After some INet searches and personal try-and-failures I succeed to rotate PNG images with preserving alpha channel transparency (semi transparency).

<?php
    $filename = 'YourFile.png';
    $rotang = 20; // Rotation angle
    $source = imagecreatefrompng($filename) or die('Error opening file '.$filename);
    imagealphablending($source, false);
    imagesavealpha($source, true);

    $rotation = imagerotate($source, $rotang, imageColorAllocateAlpha($source, 0, 0, 0, 127));
    imagealphablending($rotation, false);
    imagesavealpha($rotation, true);

    header('Content-type: image/png');
    imagepng($rotation);
    imagedestroy($source);
    imagedestroy($rotation);
?>

Don't be tricked into thinking this function will rotate the image resource handle in-place. While the documentation currently doesn't specify and return values, I've found the following return values to be true:

"Returns an image resource identifier on success, FALSE on errors. "

Simply make sure you're doing something like...
<?php
$rotated_img = imagerotate($src_img, 45, $color)
?>

-Mike

Keep in mind this function rotates counterclockwise.  if you want to go 90 degrees clockwise, you have to tell imagerotate to go 270 degrees counter-clockwise.

$angle = 360-$angle;

I've benchmarked three methods described here for rotating an image.

I used a 1600 x 1200 jpeg picture that was duplicated ten times on the disk. I looped through them to apply a CW rotation using either method.

First algorithm (the "imagesetpixel" algorithm):
<?php
function CWRotation($image)
{
    $w = imagesx($image);
    $h = imagesy($image);
    $result = @imagecreatetruecolor($h, $w);
    if($result)
    {
        for ($i = 0; $i < $w; $i++)
            for ($j = 0; $j < $h; $j++)
            {
                $ref = imagecolorat($image, $i, $j);
                imagesetpixel($result, ($h - 1) - $j, $i, $ref);
            }
    }
    return $result;
}
?>

Second algorithm (the "imagecopy" algorithm):
<?php
function CWRotation($image)
{
    $w = imagesx($image);
    $h = imagesy($image);
    $result = @imagecreatetruecolor($h, $w);
    if($result)
    {
        for ($i = 0; $i < $w; $i++)
            for ($j = 0; $j < $h; $j++)
                imagecopy($result, $image, ($h - 1) - $j, $i, $i, $j, 1, 1);
    }
    return $result;
}
?>

Third algorithm (the "imagerotate" algorithm):
<?php
function CWRotation($image)
{
    return imagerotate($image, 270, 0);
}
?>

In each step of the loop, the image is loaded, rotated then a copy is written on the disk.

Here are the results:
imagesetpixel algorithm: the 10 pictures were processed in 29 seconds
imagecopy algorithm: the 10 pictures were processed in 26 seconds
imagerotate algorithm: the 10 pictures were processed in 2 seconds (no typo here)

Writing the pictures on the disk allowed me to control the results. Every resulting picture was the same (tested with winmerge).

Conclusion: imagerotate is way faster than other "home-made" algorithms.
I ran the tests several times to see if the difference between imagesetpixel and imagecopy was introduced by my computer's independent activity, but I always got that 10% difference.

Note: the picture I used was taken from a camera, so no transparency was involved.
I ran the tests on Windows XP using PHP Version 5.3.0

An issue with imagerotate() is it might need more memory than what is available. It was failing in my script so I tried to increase it with ini_set('memory_limit', '256M') 
and everything works fine now.

About that rotating function by david at horizon-nigh not working, I found the bug.
I noticed that the x1 and y1 where not used, so, in the loop through every pixel and transform it, change

$x2 = $x * cos($theta) - $y * sin($theta);
$y2 = $x * sin($theta) + $y * cos($theta);

to:
           
$x2 = $x1 * cos($theta) - $y1 * sin($theta);
$y2 = $x1 * sin($theta) + $y1 * cos($theta);

As an alternative to the GD rotateImage() function, this is an easy way to rotate images by 90 degrees ccw *without* loosing transparency:

<?php

  function rotateImage($image) {
      $width = imagesx($image);
      $height = imagesy($image);
      $newImage= imagecreatetruecolor($height, $width);
      imagealphablending($newImage, false);
      imagesavealpha($newImage, true);
      for($w=0; $w<$width; $w++)
          for($h=0; $h<$height; $h++) {
              $ref = imagecolorat($image, $w, $h);
              imagesetpixel($newImage, $h, ($width-1)-$w, $ref);
          }
      return $newImage;
  }

?>

Hi, I've rewrote function to class with static methods. Advantage of this solution is ability to be called several times during script execution.

<?php
/*
    Unfinished imagerotate replacement. ignore_transparent is, well, ignored. :)
    Also, should have some standard functions for 90, 180 and 270 degrees, since they are quite
    easy to implement faster.
*/

if(!function_exists("imagerotate")) {
    function imagerotate(&$srcImg, $angle, $bgcolor, $ignore_transparent = 0) {
        return imagerotateEquivalent::rotate(&$srcImg, $angle, $bgcolor, $ignore_transparent);
    }
}

class imagerotateEquivalent {

    static private function rotateX($x, $y, $theta){
        return $x * cos($theta) - $y * sin($theta);
    }

    static private function rotateY($x, $y, $theta){
        return $x * sin($theta) + $y * cos($theta);
    }

    public static function rotate(&$srcImg, $angle, $bgcolor, $ignore_transparent = 0) {
        
        $srcw = imagesx($srcImg);
        $srch = imagesy($srcImg);

        if($angle == 0) return $srcImg;

        // Convert the angle to radians
        $theta = deg2rad ($angle);

        // Calculate the width of the destination image.
        $temp = array (    self::rotateX(0,     0, 0-$theta),
                        self::rotateX($srcw, 0, 0-$theta),
                        self::rotateX(0,     $srch, 0-$theta),
                        self::rotateX($srcw, $srch, 0-$theta)
                    );
        $minX = floor(min($temp));
        $maxX = ceil(max($temp));
        $width = $maxX - $minX;

        // Calculate the height of the destination image.
        $temp = array (    self::rotateY(0,     0, 0-$theta),
                        self::rotateY($srcw, 0, 0-$theta),
                        self::rotateY(0,     $srch, 0-$theta),
                        self::rotateY($srcw, $srch, 0-$theta)
                    );
        $minY = floor(min($temp));
        $maxY = ceil(max($temp));
        $height = $maxY - $minY;

        $destimg = imagecreatetruecolor($width, $height);
        imagefill($destimg, 0, 0, imagecolorallocate($destimg, 0,255, 0));

        // sets all pixels in the new image
        for($x=$minX;$x<$maxX;$x++) {
            for($y=$minY;$y<$maxY;$y++)
            {
                // fetch corresponding pixel from the source image
                $srcX = round(self::rotateX($x, $y, $theta));
                $srcY = round(self::rotateY($x, $y, $theta));
                if($srcX >= 0 && $srcX < $srcw && $srcY >= 0 && $srcY < $srch)
                {
                    $color = imagecolorat($srcImg, $srcX, $srcY );
                }
                else
                {
                    $color = $bgcolor;
                }
                imagesetpixel($destimg, $x-$minX, $y-$minY, $color);
            }
        }
        return $destimg;
    }
}

?>

I modified cbl25's function to allow it to rotate an image either clock wise or counter clock wise.

<?php
function rotateImage($image, $direction) {
    $direction = strtolower($direction);
    $degrees = $direction == 'cw' ? 270 : ($direction == 'ccw' ? 90 : NULL); 
    if(!$degrees)
        return $image;
    $width = imagesx($image);
    $height = imagesy($image);
    $side = $width > $height ? $width : $height;
    $imageSquare = imagecreatetruecolor($side, $side);
    imagecopy($imageSquare, $image, 0, 0, 0, 0, $width, $height);
    imagedestroy($image);
    $imageSquare = imagerotate($imageSquare, $degrees, 0, -1);
    $image = imagecreatetruecolor($height, $width);
    $x = $degrees == 90 ? 0 : ($height > $width ? 0 : ($side - $height));
    $y = $degrees == 270 ? 0 : ($height < $width ? 0 : ($side - $width));
    imagecopy($image, $imageSquare, 0, 0, $x, $y, $height, $width);
    imagedestroy($imageSquare);
    return $image;
}

//Usage
$image = rotateImage($image, 'cw');
$image = rotateImage($image, 'ccw');
?>

The angle in imagerotate() is the number of degrees to rotate the image anti-clockwise, but while it may seem natural to use '-90' to turn an image 90 degrees clockwise, the end result may appear on a slight angle, and may cause the rotated image to appear slightly blurred with a background or border. Excessively large angles may also present sampling issues.

The easiest way to prevent these, is to ensure all angles are between 0 and 360.

<?php
while ( $angle < 0 ) { $angle += 360; }
while ( $angle >= 360 ) { $angle -= 360; }

$rotated = imagerotate( $image, $angle, $color );
?>

I was quite surprised to see that no one had done any working rotate function. And since I needed one for my web hotel, I whipped this one up. Someone might want to expand it so that it matches imagerotate.

Also, does imagerotate really rotate things counter clockwise? Otherwise this should also be changed.

<?php
if(!function_exists("imagerotate")) {
    function imagerotate(&$srcImg, $angle, $bgcolor, $ignore_transparent = 0) {
        return imagerotateEquivalent(&$srcImg, $angle, $bgcolor, $ignore_transparent);
    }
}

/*
    Unfinished imagerotate replacement. ignore_transparent is, well, ignored. :)
    Also, should have some standard functions for 90, 180 and 270 degrees, since they are quite 
    easy to implement faster.
*/
function imagerotateEquivalent(&$srcImg, $angle, $bgcolor, $ignore_transparent = 0) 
{
    function rotateX($x, $y, $theta){
        return $x * cos($theta) - $y * sin($theta);
    }
    function rotateY($x, $y, $theta){
        return $x * sin($theta) + $y * cos($theta);
    }
    
    $srcw = imagesx($srcImg);
    $srch = imagesy($srcImg);
  
    if($angle == 0) return $srcImg;
    
    // Convert the angle to radians
    $theta = deg2rad ($angle);

    
    // Calculate the width of the destination image.
    $temp = array (    rotateX(0,     0, 0-$theta),
                    rotateX($srcw, 0, 0-$theta),
                    rotateX(0,     $srch, 0-$theta),
                    rotateX($srcw, $srch, 0-$theta)
                );
    $minX = floor(min($temp));
    $maxX = ceil(max($temp));
    $width = $maxX - $minX;
    
    // Calculate the height of the destination image.
    $temp = array (    rotateY(0,     0, 0-$theta),
                    rotateY($srcw, 0, 0-$theta),
                    rotateY(0,     $srch, 0-$theta),
                    rotateY($srcw, $srch, 0-$theta)
                );
    $minY = floor(min($temp));
    $maxY = ceil(max($temp));
    $height = $maxY - $minY;
    
    $destimg = imagecreatetruecolor($width, $height);
    imagefill($destimg, 0, 0, imagecolorallocate($destimg, 0,255, 0));

    // sets all pixels in the new image
    for($x=$minX;$x<$maxX;$x++) {
        for($y=$minY;$y<$maxY;$y++) 
        {
            // fetch corresponding pixel from the source image
            $srcX = round(rotateX($x, $y, $theta));
            $srcY = round(rotateY($x, $y, $theta));
            if($srcX >= 0 && $srcX < $srcw && $srcY >= 0 && $srcY < $srch)
            {
                $color = imagecolorat($srcImg, $srcX, $srcY );
            }
            else
            {
                $color = $bgcolor;
            }
            imagesetpixel($destimg, $x-$minX, $y-$minY, $color);
        }
    }
    
    return $destimg;
}
?>

Function to rotate images in the most common formats

<?php
function rota_img ($imagen_original,$angulo){  
    $tipo = mime_content_type($imagen_original);   
    /*
 $imagen_original ruta con referencia al servidor:
/home/tecnoval/public_html/clientes/rapidelicias/hamburgesa.jpg
     */
    $parte = explode('/',$tipo); // MIME TYPE = image/jpeg
    $ext_real  = $parte[1];
    $admitidos = array('jpeg','png','gif','webp'); // mime devuelve jpeg así sea 'jpg',

    if(in_array($ext_real,$admitidos)){
        // funciones según tipo de archivo 
        $crear['jpeg'] = 'imagecreatefromjpeg';$guardar['jpeg'] = 'imagejpeg';
        $crear['png'] = 'imagecreatefrompng';$guardar['png'] = 'imagepng';
        $crear['gif'] = 'imagecreatefromgif';$guardar['gif'] = 'imagegif';
        $crear['webp'] = 'imagecreatefromwebp';$guardar['webp'] = 'imagewebp';

        // Crear imagen desde archivo -> PE $imagen = imagecreatefromjpeg($imagen_original);
        $imagen= $crear[$ext_real]($imagen_original);

        // Rotar la imagen
        $imagen_rotada = imagerotate($imagen, $angulo, 0); // El 0 es color negro de fondo

        // Guardar imagen rotada con el mismo nombre
        $x= $guardar[$ext_real]($imagen_rotada, $imagen_original);
        if($x){echo "Imagen rotada $angulo grados ";}

        // Liberar memoria
        imagedestroy($imagen);
    }
}
?>

I wanted to draw a transparent GIF-image and show it on a page, at an angle specified in the URL:
<img src="image.php?angle=90" type="image/gif">

I used the native imagerotate() but at angles of 90, 180, etc. the old background colour would become non-transparent. Apparently, there's a bug somewhere in GD, and has been for ages.

My solution below:

<?php
$height = 100;
$width = 100;
$lsize= $width/2;
$angle= $_GET["angle"];

// avoid the bug:
if(($angle%90)==0)
  $angle+= 0.001;

$image_p = imagecreatetruecolor($width, $height);
$trans = imagecolorallocate($image_p, 254, 0, 0);
imagefill($image_p, 0, 0, $trans);
imagecolortransparent($image_p, $trans);

$black = imagecolorallocate($image_p, 1, 1, 1);
$red = imagecolorallocate($image_p, 255, 0, 0);
$white = imagecolorallocate($image_p, 255, 255, 255);

// draw something here
imageline($image_p, 3, $lsize, $lsize/2, $lsize, $black);

$image_r= imagerotate($image_p, -$angle, $trans, 0); 
$w= imagesx($image_r);
$h= imagesy($image_r);
$image_s = imagecreatetruecolor($width, $height);
imagecopyresized($image_s, $image_r, 0, 0, ($w-$width)/2, ($h-$height)/2, $width, $height, $width, $height);
$trans = imagecolorallocate($image_s, 254, 0, 0);
imagecolortransparent($image_s, $trans);
imagegif($image_s);
?>

I couldn't get the replacements from 'the dot thawk' or 'pilot' to work for some reason, so here's my own replacement. It uses ImageMagick; binary must be installed, and you may need to modify the search path. (I didn't use PHP's ImageMagick support for my own reasons.)

<?php
    /**
     * imagerotate()
     * Debian php5-gd packages do not include imagerotate() due to some convoluted reason.
     *
     * @param int $angle - same as PHP builtin function
     * @param $bgd_color - not implemented, apparently always #FFFFFF
     *
     * @return same as PHP builtin function
     */
    if ( !function_exists( 'imagerotate' ) ) {

        function imagerotate( $source_image, $angle, $bgd_color ) {

            $angle = 360-$angle; // GD rotates CCW, imagick rotates CW

            foreach ( array( '/usr/bin', '/usr/local/bin', '/opt/local/bin', '/sw/bin' ) as $path ) {

                if ( file_exists( $path . '/convert' ) ) {
                    $imagick = $path . '/convert';
                    if ( $path == '/opt/local/bin' ) {
                        $imagick = 'DYLD_LIBRARY_PATH="" ' . $imagick; // some kind of conflict with MacPorts and MAMP
                    }
                    break;
                }

            }

            if ( !isset( $imagick ) ) {

                //trigger_error( 'imagerotate(): could not find imagemagick binary, original image returned', E_USER_WARNING );
                return $source_image;

            }

            $file1 = '/tmp/imagick_' . rand( 10000,99999 ) . '.png';
            $file2 = '/tmp/imagick_' . rand( 10000,99999 ) . '.png';

            if ( @imagepng( $source_image, $file1 ) ) {

                exec( $imagick . ' -rotate ' . $angle . ' ' . $file1 . ' ' . $file2 );

                if ( file_exists( $file2 ) ) {

                    $new_image = imagecreatefrompng( $file2 );
                    unlink( $file1 );
                    unlink( $file2 );
                    return $new_image;

                } else {

                    //trigger_error( 'imagerotate(): imagemagick conversion failed, original image returned', E_USER_WARNING );
                    return $source_image;

                }

            } else {

                //trigger_error( 'imagerotate(): could not write to ' . $file1 . ', original image returned', E_USER_WARNING );
                return $source_image;

            }

        }

    }
?>

In response to pilot at myupb dot com on 31-May-2008 02:23
---

I am not sure why you would be defining your own PI, instead of using the built-in constant, and why you do the degrees to radian conversion manually. There might be a speed issue, however here is the exact same code with that small difference.

<?php
if(!function_exists("imagerotate")) {
    function imagerotate(&$srcImg, $angle, $transparentColor = null) {
        $srcw = imagesx($srcImg);
        $srch = imagesy($srcImg);
       
        if($angle == 0) return $srcImg;
       
        // Convert the angle to radians
        $theta = deg2rad ($angle);
       
        // Get the origin (center) of the image
        $originx = $srcw / 2;
        $originy = $srch / 2;
       
        // The pixels array for the new image
        $pixels = array();
        $minx = 0;
        $maxx = 0;
        $miny = 0;
        $maxy = 0;
        $dstw = 0;
        $dsth = 0;
       
        // Loop through every pixel and transform it
        for($x=0;$x<$srcw;$x++) {
            for($y=0;$y<$srch;$y++) {
                list($x1, $y1) = translateCoordinate($originx, $originy, $x, $y, false);
               
                $x2 = $x * cos($theta) - $y * sin($theta);
                $y2 = $x * sin($theta) + $y * cos($theta);
               
                // Store the pixel color
                $pixels[] = array($x2, $y2, imagecolorat($srcImg, $x, $y));
               
                // Check our boundaries
                if($x2 > $maxx) $maxx = $x2;
                if($x2 < $minx) $minx = $x2;
                if($y2 > $maxy) $maxy = $y2;
                if($y2 < $miny) $miny = $y2;
            }
        }
       
        // Determine the new image size
        $dstw = $maxx - $minx + 1;
        $dsth = $maxy - $miny + 1;
       
        // Create our new image
        $dstImg = imagecreatetruecolor($dstw, $dsth);
       
        // Fill the background with our transparent color
        if($transparentColor == null) $transparentColor = imagecolorallocate($dstImg, 1, 2, 3);
        imagecolortransparent($dstImg, $transparentColor);
        imagefilledrectangle($dstImg, 0, 0, $dstw + 1, $dsth + 1, $transparentColor);
       
        // Get the new origin
        $neworiginx = -$minx;
        $neworiginy = -$miny;
       
        // Fill in the pixels
        foreach($pixels as $data) {
            list($x, $y, $color) = $data;
            list($newx, $newy) = translateCoordinate($neworiginx, $neworiginy, $x, $y);
            imagesetpixel($dstImg, $newx, $newy, $color);
        }
       
        return $dstImg;
    }
   
    /**
     * Translates from mathematical coordinate system to computer coordinate system using
     * origin coordinates from the computer system or visa versa
     *
     * @param int $originx
     * @param int $originy
     * @param int $x
     * @param int $y
     * @param bool $toComp
     * @return array(int $x, int $y)
     */
    function translateCoordinate($originx, $originy, $x, $y, $toComp=true) {
        if($toComp) {
            $newx = $originx + $x;
            $newy = $originy - $y;
        } else {
            $newx = $x - $originx;
            $newy = $originy - $y;
        }
       
        return array($newx, $newy);
    }
}
?>

For those who are looking for the GD library for Ubuntu, I let a copy on my server:  http://www.cartefoi.net/compl_compl_2.php
It was pretty hard to find, somebody gaveme his. Thanks a lot to him (message up).

Sorry, the previous class contains an error, the original image after the rotation 1px move on and get the unwanted "border".

After a careful reading of the local debate, I am using the tip from Dave Richards wrote a new function. With its images can be rotated only 90 ° (default), 180 ° and 270 °, but one rarely needs more ...

The function returns False, or rotated image 
<?php
if(!function_exists("imagerotate")) {
    function imagerotate($srcImg, $angle, $bgcolor, $ignore_transparent = 0) {
        return rotateImage($srcImg, $angle);
    }
}

function rotateImage($img1, $rec) {
    $wid = imagesx($img1);
    $hei = imagesy($img1);
    switch($rec) {
        case 270:
            $img2 = @imagecreatetruecolor($hei, $wid);
        break;
        case 180:
            $img2 = @imagecreatetruecolor($wid, $hei);
        break;
        default :
            $img2 = @imagecreatetruecolor($hei, $wid);
    }
    if($img2) {
        for($i = 0;$i < $wid; $i++) {
            for($j = 0;$j < $hei; $j++) {
                $ref = imagecolorat($img1,$i,$j);
                switch($rec) {
                    case 270:
                        if(!@imagesetpixel($img2, ($hei - 1) - $j, $i, $ref)){
                            return false;
                        }
                    break;
                    case 180:
                        if(!@imagesetpixel($img2, $i, ($hei - 1) - $j, $ref)) {
                            return false;
                        }
                    break;
                    default:
                        if(!@imagesetpixel($img2, $j, ($wid - 1) - $i, $ref)) {
                            return false;
                        }
                }
            }
        }
        return $img2;
    }
    return false;
}
?>

Petr

I'm now using david's code with these changes to get a transparent effect.

I exec convert 2 times:

<?php
exec( $imagick . ' -background \'#555555\' -rotate ' . $angle . ' ' . $file1 . ' ' . $file2 );
exec( $imagick . ' -transparent \'#555555\' -transparent-color \'#555555\' ' . $file2 . ' ' . $file3 );
?>

and then use this after I rotated $output:
<?php imagecolortransparent($output, imagecolorallocatealpha($output, 85, 85, 85, 127)); ?>

Here is a simple function to rotate a non-square image 90 degrees clockwise.

<?php
function rotateImage($imageResource)
{
    $width = imagesx($imageResource);
    $height = imagesy($imageResource);
    $side = $width > $height ? $width : $height;
    $squareImage = imagecreatetruecolor($side, $side);
    imagecopy($squareImage,$imageResource,0,0,0,0,$width,$height);
    $squareImage = imagerotate($squareImage,270,0,-1);
    $imageResource = imagecreatetruecolor($height, $width);
    $x = $height > $width ? 0 : $side - $height;
    imagecopy($imageResource,$squareImage,0,0,$x,0,$height,$width);
    return $imageResource;
}
?>

Shortly after posting I was informed about a very nice algorithm for calculating the new pixel position for a rotation and thought it would be a nice improvement. Hopes this helps anyone needing a full implementation.

Note: I tested the function on a 3Mb photo and it seemed to work perfectly with no quality loss.

<?php
if(!function_exists("imagerotate")) {
    function imagerotate(&$srcImg, $angle, $transparentColor = null) {
        $srcw = imagesx($srcImg);
        $srch = imagesy($srcImg);
        
        if($angle == 0) return $srcImg;
        
        // Convert the angle to radians
        $pi = 3.141592654;
        $theta = $angle * $pi / 180;
        
        // Get the origin (center) of the image
        $originx = $srcw / 2;
        $originy = $srch / 2;
        
        // The pixels array for the new image
        $pixels = array();
        $minx = 0;
        $maxx = 0;
        $miny = 0;
        $maxy = 0;
        $dstw = 0;
        $dsth = 0;
        
        // Loop through every pixel and transform it
        for($x=0;$x<$srcw;$x++) {
            for($y=0;$y<$srch;$y++) {
                list($x1, $y1) = translateCoordinate($originx, $originy, $x, $y, false);
                
                $x2 = $x * cos($theta) - $y * sin($theta);
                $y2 = $x * sin($theta) + $y * cos($theta);
                
                // Store the pixel color
                $pixels[] = array($x2, $y2, imagecolorat($srcImg, $x, $y));
                
                // Check our boundaries
                if($x2 > $maxx) $maxx = $x2;
                if($x2 < $minx) $minx = $x2;
                if($y2 > $maxy) $maxy = $y2;
                if($y2 < $miny) $miny = $y2;
            }
        }
        
        // Determine the new image size
        $dstw = $maxx - $minx + 1;
        $dsth = $maxy - $miny + 1;
        
        // Create our new image
        $dstImg = imagecreatetruecolor($dstw, $dsth);
        
        // Fill the background with our transparent color
        if($transparentColor == null) $transparentColor = imagecolorallocate($dstImg, 1, 2, 3);
        imagecolortransparent($dstImg, $transparentColor);
        imagefilledrectangle($dstImg, 0, 0, $dstw + 1, $dsth + 1, $transparentColor);
        
        // Get the new origin
        $neworiginx = -$minx;
        $neworiginy = -$miny;
        
        // Fill in the pixels
        foreach($pixels as $data) {
            list($x, $y, $color) = $data;
            list($newx, $newy) = translateCoordinate($neworiginx, $neworiginy, $x, $y);
            imagesetpixel($dstImg, $newx, $newy, $color);
        }
        
        return $dstImg;
    }
    
    /**
     * Translates from mathematical coordinate system to computer coordinate system using
     * origin coordinates from the computer system or visa versa
     *
     * @param int $originx
     * @param int $originy
     * @param int $x
     * @param int $y
     * @param bool $toComp
     * @return array(int $x, int $y)
     */
    function translateCoordinate($originx, $originy, $x, $y, $toComp=true) {
        if($toComp) {
            $newx = $originx + $x;
            $newy = $originy - $y;
        } else {
            $newx = $x - $originx;
            $newy = $originy - $y;
        }
        
        return array($newx, $newy);
    }
}
?>

Thanks to the people who contributed the code for the 90 180 and 270 rotations. I needed a full implementation however so I wrote one. By no mean do I think this is the best way of doing it, I just whipped this together for myself. Seems to work good for me.

Note: I didn't want the image to be shrunk when rotating so this implementation will keep the size of the original image but just rotate it.

<?php
if(!function_exists("imagerotate")) {
    function imagerotate(&$srcImg, $angle, $transparentColor = null) {
        $srcw = imagesx($srcImg);
        $srch = imagesy($srcImg);
        
        if($angle == 0) return $srcImg;
        
        // Convert the angle to radians
        $pi = 3.141592654;
        $theta = $angle * $pi / 180;
        
        // Get the origin (center) of the image
        $originx = $srcw / 2;
        $originy = $srch / 2;
        
        // The pixels array for the new image
        $pixels = array();
        $minx = 0;
        $maxx = 0;
        $miny = 0;
        $maxy = 0;
        $dstw = 0;
        $dsth = 0;
        
        // Loop through every pixel and transform it
        for($x=0;$x<$srcw;$x++) {
            for($y=0;$y<$srch;$y++) {
                list($x1, $y1) = translateCoordinate($originx, $originy, $x, $y, false);
                
                $theta1 = 0;
                $noTranslate = false;
                
                // Determine the angle of original point
                if($x1 > 0 && $y1 > 0) {
                    // Quadrant 1
                    $theta1 = atan($y1/$x1);
                } elseif($x1 < 0 && $y1 > 0) {
                    // Quadrant 2
                    $theta1 = $pi - atan($y1/abs($x1));
                } elseif($x1 < 0 && $y1 < 0) {
                    // Quadrant 3
                    $theta1 = $pi + atan($y1/$x1);
                } elseif($x1 > 0 && $y1 < 0) {
                    // Quadrant 4
                    $theta1 = 2 * $pi - atan(abs($y1)/$x1);
                } elseif($x1 == 0 && $y1 > 0) {
                    $theta1 = $pi / 2;
                } elseif($x1 == 0 && $y1 < 0) {
                    $theta1 = 3 * $pi / 2;
                } elseif($x1 > 0 && $y1 == 0) {
                    $theta1 = 0;
                } elseif($x1 < 0 && $y1 == 0) {
                    $theta1 = $pi;
                } else {
                    // Only case left should be $x1 == 0 && $y1 == 0
                    $noTranslate = true;
                }
                
                // Translate the position
                if(!$noTranslate) {
                    // Calculate the new angle
                    $theta2 = $theta1 + $theta;
                    
                    // Make sure theta2 is in between 0 - 2pi
                    while($theta2 < 0) $theta2 += 2 * $pi;
                    while($theta2 > (2 * $pi)) $theta2 -= 2 * $pi;
                    
                    $radius = sqrt($x1*$x1 + $y1*$y1);
                    
                    $x2 = ($radius * cos($theta2));
                    $y2 = ($radius * sin($theta2));
                } else {
                    $x2 = $x1;
                    $y2 = $y1;
                }
                
                // Store the pixel color
                $pixels[] = array($x2, $y2, imagecolorat($srcImg, $x, $y));
                
                // Check our boundaries
                if($x2 > $maxx) $maxx = $x2;
                if($x2 < $minx) $minx = $x2;
                if($y2 > $maxy) $maxy = $y2;
                if($y2 < $miny) $miny = $y2;
            }
        }
        
        // Determine the new image size
        $dstw = $maxx - $minx + 1;
        $dsth = $maxy - $miny + 1;
        
        // Create our new image
        $dstImg = imagecreatetruecolor($dstw, $dsth);
        
        // Fill the background with our transparent color
        if($transparentColor == null) $transparentColor = imagecolorallocate($dstImg, 1, 2, 3);
        imagecolortransparent($dstImg, $transparentColor);
        imagefilledrectangle($dstImg, 0, 0, $dstw + 1, $dsth + 1, $transparentColor);
        
        // Get the new origin
        $neworiginx = -$minx;
        $neworiginy = -$miny;
        
        // Fill in the pixels
        foreach($pixels as $data) {
            list($x, $y, $color) = $data;
            list($newx, $newy) = translateCoordinate($neworiginx, $neworiginy, $x, $y);
            imagesetpixel($dstImg, $newx, $newy, $color);
        }
        
        return $dstImg;
    }
    
    /**
     * Translates from mathematical coordinate system to computer coordinate system using
     * origin coordinates from the computer system or visa versa
     *
     * @param int $originx
     * @param int $originy
     * @param int $x
     * @param int $y
     * @param bool $toComp
     * @return array(int $x, int $y)
     */
    function translateCoordinate($originx, $originy, $x, $y, $toComp=true) {
        if($toComp) {
            $newx = $originx + $x;
            $newy = $originy - $y;
        } else {
            $newx = $x - $originx;
            $newy = $originy - $y;
        }
        
        return array($newx, $newy);
    }
}
?>

otimized integration from 23-Feb-2007 04:21
just put it anywhere you like to use imagerotate with 90, 180, 270 degrees.
<?php
if(!function_exists("imagerotate")) {
    function imagerotate($src_img, $angle) {
        $src_x = imagesx($src_img);
        $src_y = imagesy($src_img);
        if ($angle == 180) {
            $dest_x = $src_x;
            $dest_y = $src_y; }
        elseif ($src_x <= $src_y) {
            $dest_x = $src_y;
            $dest_y = $src_x; }
        elseif ($src_x >= $src_y) {
            $dest_x = $src_y;
            $dest_y = $src_x; }
        
        $rotate=imagecreatetruecolor($dest_x,$dest_y);
        imagealphablending($rotate, false);
               
        switch ($angle) {
            case 270:
                for ($y = 0; $y < ($src_y); $y++) {
                    for ($x = 0; $x < ($src_x); $x++) {
                        $color = imagecolorat($src_img, $x, $y);
                        imagesetpixel($rotate, $dest_x - $y - 1, $x, $color); }}
                break;
            case 90:
                for ($y = 0; $y < ($src_y); $y++) {
                    for ($x = 0; $x < ($src_x); $x++) {
                        $color = imagecolorat($src_img, $x, $y);
                        imagesetpixel($rotate, $y, $dest_y - $x - 1, $color); }}
                break;
            case 180:
                for ($y = 0; $y < ($src_y); $y++) {
                    for ($x = 0; $x < ($src_x); $x++) {
                        $color = imagecolorat($src_img, $x, $y);
                        imagesetpixel($rotate, $dest_x - $x - 1, $dest_y - $y - 1, $color); }}
                break;
            default: $rotate = $src_img; }
        return $rotate; }}
?>

imagerotate seems to be very fussy about handling transparency when copymerging onto another image. You can use the GD library's other transparency features to cover up the fact imagerotate gets it wrong HOWEVER it will only work if the top-left corner of the image is transparent at all rotations, so make the image a little bigger than it needs to be. This has been tested with png32 but does not work entirely for png8, as a phenomena creates noise around the rotated image.

<?php

$imgImage = imagecreatefrompng("image.png");
$colBlack = imagecolorallocate($imgImage, 0, 0, 0);
$imgImage = imagerotate($imgImage, 360 - $intHeading, 0);
imagefill($imgImage, 0, 0, $colBlack);
imagecolortransparent($imgImage, $colBlack);
imagecopymerge($imgOriginalImage , $imgImage, $intX, $intY, 0, 0, $intHeight, $intWidth, 100);
imagedestroy($imgImage);

?>

Note. $intHeading is in degrees clockwise :)

<?php
function rotate($degrees)
    {
        if(function_exists("imagerotate"))
            $this->image = imagerotate($this->image, $degrees, 0);
        else
        {
            function imagerotate($src_img, $angle) 
            {
                $src_x = imagesx($src_img);
                $src_y = imagesy($src_img);
                if ($angle == 180)
                {
                    $dest_x = $src_x;
                    $dest_y = $src_y;
                } 
                elseif ($src_x <= $src_y) 
                {
                    $dest_x = $src_y;
                    $dest_y = $src_x;
                } 
                elseif ($src_x >= $src_y)  
                {
                    $dest_x = $src_y;
                    $dest_y = $src_x;
                }
                
                $rotate=imagecreatetruecolor($dest_x,$dest_y);
                imagealphablending($rotate, false);
                
                switch ($angle) 
                {
                    case 270:
                        for ($y = 0; $y < ($src_y); $y++) 
                        {
                            for ($x = 0; $x < ($src_x); $x++) 
                            {
                                $color = imagecolorat($src_img, $x, $y);
                                imagesetpixel($rotate, $dest_x - $y - 1, $x, $color);
                            }
                        }
                        break;
                    case 90:
                        for ($y = 0; $y < ($src_y); $y++) 
                        {
                            for ($x = 0; $x < ($src_x); $x++) 
                            {
                                $color = imagecolorat($src_img, $x, $y);
                                imagesetpixel($rotate, $y, $dest_y - $x - 1, $color);
                            }
                        }
                        break;
                    case 180:
                        for ($y = 0; $y < ($src_y); $y++) 
                        {
                            for ($x = 0; $x < ($src_x); $x++) 
                            {
                                $color = imagecolorat($src_img, $x, $y);
                                imagesetpixel($rotate, $dest_x - $x - 1, $dest_y - $y - 1, $color);
                            }
                        }
                        break;
                    default: $rotate = $src_img;
                };
                return $rotate;
            }
            $this->image = imagerotate($this->image, $degrees);
        }
    }
?>

I liked the rotateImageBicubic function implemented by darren at lucidtone dot com. But it just snipped off the parts of the image that were outside the original image.

I fixed this, even though I admit that my solution is a bit naive. But it might come in handy for somebody.

Also his bicubic implementation was broken on my machine so I left it out, if you need it just copy and paste it from above.

<?php

// $src_img - a GD image resource
// $angle - degrees to rotate clockwise, in degrees
// returns a GD image resource
// USAGE:
// $im = imagecreatefrompng('test.png');
// $im = imagerotate($im, 15);
// header('Content-type: image/png');
// imagepng($im);
function imageRotate($src_img, $angle, $bicubic=false) {
  
   // convert degrees to radians
   $angle = $angle + 180;
   $angle = deg2rad($angle);
  
   $src_x = imagesx($src_img);
   $src_y = imagesy($src_img);
  
   $center_x = floor($src_x/2);
   $center_y = floor($src_y/2);

   $cosangle = cos($angle);
   $sinangle = sin($angle);

   $corners=array(array(0,0), array($src_x,0), array($src_x,$src_y), array(0,$src_y));

   foreach($corners as $key=>$value) {
     $value[0]-=$center_x;        //Translate coords to center for rotation
     $value[1]-=$center_y;
     $temp=array();
     $temp[0]=$value[0]*$cosangle+$value[1]*$sinangle;
     $temp[1]=$value[1]*$cosangle-$value[0]*$sinangle;
     $corners[$key]=$temp;    
   }
   
   $min_x=1000000000000000;
   $max_x=-1000000000000000;
   $min_y=1000000000000000;
   $max_y=-1000000000000000;
   
   foreach($corners as $key => $value) {
     if($value[0]<$min_x)
       $min_x=$value[0];
     if($value[0]>$max_x)
       $max_x=$value[0];
   
     if($value[1]<$min_y)
       $min_y=$value[1];
     if($value[1]>$max_y)
       $max_y=$value[1];
   }

   $rotate_width=round($max_x-$min_x);
   $rotate_height=round($max_y-$min_y);

   $rotate=imagecreatetruecolor($rotate_width,$rotate_height);
   imagealphablending($rotate, false);
   imagesavealpha($rotate, true);

   //Reset center to center of our image
   $newcenter_x = ($rotate_width)/2;
   $newcenter_y = ($rotate_height)/2;

   for ($y = 0; $y < ($rotate_height); $y++) {
     for ($x = 0; $x < ($rotate_width); $x++) {
       // rotate...
       $old_x = round((($newcenter_x-$x) * $cosangle + ($newcenter_y-$y) * $sinangle))
         + $center_x;
       $old_y = round((($newcenter_y-$y) * $cosangle - ($newcenter_x-$x) * $sinangle))
         + $center_y;
      
       if ( $old_x >= 0 && $old_x < $src_x
             && $old_y >= 0 && $old_y < $src_y ) {

           $color = imagecolorat($src_img, $old_x, $old_y);
       } else {
         // this line sets the background colour
         $color = imagecolorallocatealpha($src_img, 255, 255, 255, 127);
       }
       imagesetpixel($rotate, $x, $y, $color);
     }
   }
   
  return($rotate);
}

?>

Here's a function that implements right angle (multiplicity of 90 degs - 90, 180, 270) rotation if you need one but lacks native imagerotate() or you don't want non-square images to be scaled down as with imagerotate(). As you probably noticed it's not self contained function, as 180 rotation is handled by ImageFlip() function to gain the performance. The ImageFlip() function used is published here: http://php.net/imagecopy in the comment of mine placed on  05-Jan-2005 04:30

Please note: that in case of 0 degrees rotation handle to imgSrc is returned which may lead to problems if you imagedestroy() it undonditionaly. To solve that you shall add imagecopy($imgDest, $imgSrc, 0,0, 0,0,$srcX, $srcY)  in proper place which I have intentionally ommited to save memory resources

<?php

// $imgSrc - GD image handle of source image
// $angle - angle of rotation. Needs to be positive integer
// angle shall be 0,90,180,270, but if you give other it
// will be rouned to nearest right angle (i.e. 52->90 degs,
// 96->90 degs)
// returns GD image handle of rotated image.
function ImageRotateRightAngle( $imgSrc, $angle )
{
    // ensuring we got really RightAngle (if not we choose the closest one)
    $angle = min( ( (int)(($angle+45) / 90) * 90), 270 );

    // no need to fight
    if( $angle == 0 )
        return( $imgSrc );

    // dimenstion of source image
    $srcX = imagesx( $imgSrc );
    $srcY = imagesy( $imgSrc );

    switch( $angle )
        {
        case 90:
            $imgDest = imagecreatetruecolor( $srcY, $srcX );
            for( $x=0; $x<$srcX; $x++ )
                for( $y=0; $y<$srcY; $y++ )
                    imagecopy($imgDest, $imgSrc, $srcY-$y-1, $x, $x, $y, 1, 1);
            break;

        case 180:
            $imgDest = ImageFlip( $imgSrc, IMAGE_FLIP_BOTH );
            break;

        case 270:
            $imgDest = imagecreatetruecolor( $srcY, $srcX );
            for( $x=0; $x<$srcX; $x++ )
                for( $y=0; $y<$srcY; $y++ )
                    imagecopy($imgDest, $imgSrc, $y, $srcX-$x-1, $x, $y, 1, 1);
            break;
        }

    return( $imgDest );
}
?>

imagerotate does not preserve the alpha channel, so if you want to rotate a PNG you need to get 
creative. I don't see any command to retrieve the alpha information from an image (as far as 
I could see,) so you'll have to do a bit of manual labor before hand. In my case I created a 
second PNG file with the alpha saved as RGB data and manually "copied" the data from source 
to destination:

<?php
    function alpha_rotate($dst,$src,$rotate,$offsetX,$offsetY){

        $top = imagecreatefrompng("image_processing/shadow.png");
        $top_alpha = imagecreatefrompng("image_processing/shadow_alpha.png");
                
        imagecopyresampled($top,$src,0,0,0,0,100,100,100,100);
                
        $top = imagerotate($top,$rotate,0x000000);
        $top_alpha = imagerotate($top_alpha,$rotate,0x000000);
    
    
        for ($theX=0;$theX<imagesx($top);$theX++){
            for ($theY=0;$theY<imagesy($top);$theY++){
    
                $rgb = imagecolorat($top,$theX,$theY);
                $r = ($rgb >> 16) & 0xFF;
                $g = ($rgb >> 8) & 0xFF;
                $b = $rgb & 0xFF;
                
                $rgb = imagecolorat($top_alpha,$theX,$theY);
                $a = $rgb & 0xFF;
                $a = 127-floor($a/2);

                $myColor = imagecolorallocatealpha($top,$r,$g,$b,$a);
                imagesetpixel($dst,($theX+$offsetX),($theY+$offsetY),$myColor);    
            }
        }
    }
?>

Note, of the rotate functions below, only beau's worked for me. Not sure if it is because of my source image, but upon rotating, the background became blue. 

The code snippet below is what I used to prepare UPS shipping labels. The UPS xml api will return a base64 encoded gif, but it is sideways, so as to print on the top half of a 8.5x11 "letter" page. We are saving it in a database and sending it to a label printer, so needed it rotated...

The other code further down didn't make the background blue, however it did make the image leave the canvas by about 50%. Additionally, the imagecolorallocate() function has an example describing how to set the background color. That didn't work for me. Even though the blue BG is #0000ff, using that code didn't work, I had to use the transparent trick below. 

The GD image functions may be in their infancy, however are a great reminder to me how powerful php is! Also, ubuntu apache users will need to apt-get install php5-gd to get the gd functions.

<?php
        $image=imagecreatefromgif( 'data://text/plain;base64,'.$this->shipmentLabelGraphicImage );
        $image=$this->rotateImage($image, 90); // Note: rotateImage turns our background to blue!
        $blue = imagecolorallocate($image, 0, 0, 255);
        imagecolortransparent($image, $blue);    // make blue transparent, so image isn't goofy
        imagepng($image,$this->shipmentTrackingNumber.".png");
        echo '<P><img src="'.$this->shipmentTrackingNumber.'.png"></P>';
        imagedestroy($image);
?>

And you can add a transparent background to you png image by change the 
<?php
    if ($ignore_transparent == 0) {
        imagefill($destimg, 0, 0, imagecolorallocatealpha($destimg, 255,255, 255, 127));
        imagesavealpha($destimg, true);
    }
?>
to
<?php
    if ($ignore_transparent == 0) {
        $temp = imagecolorallocatealpha($destimg, 255,255, 255, 127);
        imagefill($destimg, 0, 0, $temp);
        //if set the default color or white or magic pink then use transparent color
        if ( ($bgcolor == 0) || ($bgcolor == 16777215) || ($bgcolor == 16711935) )  {
            $bgcolor = $temp;
        }
        imagesavealpha($destimg, true);
    }
?>

If you're going to do a rotation by hand instead of using the imagerotate function, you can use the imagecopy function to copy a single pixel instead of determining the pixel color and setting the pixel.  Not only is this shorter and cleaner, but it also works with images that are not truecolor (on my system, imagecolorat fails with an image that is not truecolor).

with large file, where imagerotate is missing, you can use, when possible "convert" command from ImageMagick. Here is a sample script.

<?php

error_reporting(E_ALL);

header("Content-type: image/png");

$file = 'images/test/imgp2498.jpg';

image_rotate_with_convert($file, 90);

function image_rotate_with_convert($file, $angle){
    passthru("convert -rotate $angle $file -");
}

?>

The following is potentially useful. It extracts the central largest circle of an image into a square of specified size, and optionally rotates it. The rest of the square is made transparent, so useful for drawing over other images. I've named it after binocular effect because on some old TV shows whenever they show someone looking through binoculars the screen shows a big circular image with black edges.

<?php

function image_binocular_effect($src, $bearing, $out_square) {
 // the source image is resampled to fit within the specified square, and rotated clockwise by bearing.
 // the largest circle within the image is retained, the rest made transparent.
 $out = imagecreatetruecolor($out_square, $out_square);
 $width=imagesx($src);
 $height=imagesy($src);
 $square=min($width, $height);
 imagecopyresampled($out, $src, 0, 0, ($width - $square)/2 , ($height - $square)/2, $out_square, $out_square, $square, $square);

 $mask = imagecreatetruecolor($out_square, $out_square);
 $black = ImageColorAllocate ($mask, 0, 0, 0);
 $white = ImageColorAllocate ($mask, 255, 255, 255);
 imagefilledrectangle($mask , 0, 0, $out_square, $out_square, $white);
 $centrexy=$out_square / 2;
 imagefilledellipse($mask, $centrexy, $centrexy, $out_square, $out_square, $black);
 ImageColorTransparent($mask, $black);
 imagecopymerge($out, $mask,  0, 0, 0, 0, $out_square, $out_square, 100);
 if ($bearing != 0) {
  $rotated_img=imagerotate($out , 360-$bearing, $white);
  // take off only the rotated width
  $rotated_map_width = imagesx($rotated_img);
  $rotated_map_height = imagesy($rotated_img);
  imagecopy($out, $rotated_img, 0, 0, ($rotated_map_width - $out_square) / 2, ($rotated_map_height - $out_square) / 2, $out_square, $out_square);
  }
 ImageColorTransparent($out, $white);
 return $out;
}

// Create a sample image to demonstrate the effect, but looks much better on real photos.

$src = imagecreatetruecolor(200, 50);
imagefilledrectangle($src, 0, 0, 200, 50, imagecolorallocate($src, 255, 255, 255));
ImageString($src, 3, 10, 10, "This is a sample image to illustrate the binocular effect", imagecolorallocate($im, 192, 0, 0));
$img=image_binocular_effect($src, 72, 50);
ImagePNG($img,"test.png");

?>

Here's a neat function for those of us who don't have imagerotate() on our servers.  It's based on a comment from ron at korving dot demon dot nl on the manual page for imagecopyresampled.

I'm still not 100% on coping with transparency, but this function seems to cope okay.  It doesn't resize to fit within bounds, it just rotates and you lose anything outside the image box.  

The bicubic mode is slooow.

If you want to be able to change the background colour, pass in a colour and use it where indicated.  The line I used just sets it transparent.

<?php
// $src_img - a GD image resource
// $angle - degrees to rotate clockwise, in degrees
// returns a GD image resource
// USAGE:
// $im = imagecreatefrompng('test.png');
// $im = imagerotate($im, 15);
// header('Content-type: image/png');
// imagepng($im);
function imageRotateBicubic($src_img, $angle, $bicubic=false) {
    
    // convert degrees to radians
    $angle = $angle + 180;
    $angle = deg2rad($angle);
    
    $src_x = imagesx($src_img);
    $src_y = imagesy($src_img);
    
    $center_x = floor($src_x/2);
    $center_y = floor($src_y/2);
    
    $rotate = imagecreatetruecolor($src_x, $src_y);
    imagealphablending($rotate, false);
    imagesavealpha($rotate, true);

    $cosangle = cos($angle);
    $sinangle = sin($angle);
    
    for ($y = 0; $y < $src_y; $y++) {
      for ($x = 0; $x < $src_x; $x++) {
    // rotate...
    $old_x = (($center_x-$x) * $cosangle + ($center_y-$y) * $sinangle)
      + $center_x;
    $old_y = (($center_y-$y) * $cosangle - ($center_x-$x) * $sinangle)
      + $center_y;
    
    if ( $old_x >= 0 && $old_x < $src_x
         && $old_y >= 0 && $old_y < $src_y ) {
      if ($bicubic == true) {
        $sY  = $old_y + 1;
        $siY  = $old_y;
        $siY2 = $old_y - 1;
        $sX  = $old_x + 1;
        $siX  = $old_x;
        $siX2 = $old_x - 1;
        
        $c1 = imagecolorsforindex($src_img, imagecolorat($src_img, $siX, $siY2));
        $c2 = imagecolorsforindex($src_img, imagecolorat($src_img, $siX, $siY));
        $c3 = imagecolorsforindex($src_img, imagecolorat($src_img, $siX2, $siY2));
        $c4 = imagecolorsforindex($src_img, imagecolorat($src_img, $siX2, $siY));
        
        $r = ($c1['red']  + $c2['red']  + $c3['red']  + $c4['red']  ) << 14;
        $g = ($c1['green'] + $c2['green'] + $c3['green'] + $c4['green']) << 6;
        $b = ($c1['blue']  + $c2['blue']  + $c3['blue']  + $c4['blue'] ) >> 2;
        $a = ($c1['alpha']  + $c2['alpha']  + $c3['alpha']  + $c4['alpha'] ) >> 2;
        $color = imagecolorallocatealpha($src_img, $r,$g,$b,$a);
      } else {
        $color = imagecolorat($src_img, $old_x, $old_y);
      }
    } else {
          // this line sets the background colour
      $color = imagecolorallocatealpha($src_img, 255, 255, 255, 127);
    }
    imagesetpixel($rotate, $x, $y, $color);
      }
    }
    return $rotate;
}
?>

I had a problem with a png image containing transparent colors, but found a solution that works for me.
So i thought i might share it:

<?php
$angle=45;
$im=imagerotate($im, $angle, -1);
imagealphablending($im, true);
imagesavealpha($im, true);
?>

Note that the image resource returned from the imagerotate() function is an entirely different image resource from your original one.

<?php

$res = imagecreatetruecolor(100, 200);
$res2 = imagerotate($res, 90, 0);
var_dump($res, $res2);

?>

This will output:

resource(2, gd)
resource(3, gd)

// angle in degrees, clockwise, allowed values from 0 to 360 degrees
    function rotateImage ($image, $angle)
    {
        if ( ($angle < 0) || ($angle > 360) )
        {
            exit ("Error, angle passed out of range: [0,360]");
        }
        
        $width    = imagesx ($image);
        $height    = imagesy ($image);
        
        $dstImage = imagecreatetruecolor ($width, $height);
        
        if ( ($angle == 0) || ($angle == 360) )
        {
            // Just copy image to output:
            imagecopy ($dstImage, $image, 0, 0, 0, 0, $width, $height);
        }
        else
        {
            $centerX = floor ($width / 2);
            $centerY = floor ($height / 2);
            
            // Run on all pixels of the destination image and fill them:
            for ($dstImageX = 0; $dstImageX < $width; $dstImageX++)
            {
                for ($dstImageY = 0; $dstImageY < $height; $dstImageY++)
                {
                    // Calculate pixel coordinate in coordinate system centered at the image center:
                    $x = $dstImageX - $centerX;
                    $y = $centerY - $dstImageY;
                    
                    if ( ($x == 0) && ($y == 0) )
                    {
                        // We are in the image center, this pixel should be copied as is:
                        $srcImageX = $x;
                        $srcImageY = $y;
                    }
                    else
                    {
                        $r = sqrt ($x * $x + $y * $y); // radius - absolute distance of the current point from image center

                        $curAngle = asin ($y / $r); // angle of the current point [rad]
                        
                        if ($x < 0)
                        {
                            $curAngle = pi () - $curAngle;
                        }
                        
                        $newAngle = $curAngle + $angle * pi () / 180; // new angle [rad]

                        // Calculate new point coordinates (after rotation) in coordinate system at image center
                        $newXRel = floor ($r * cos ($newAngle));
                        $newYRel = floor ($r * sin ($newAngle));
                        
                        // Convert to image absolute coordinates
                        $srcImageX = $newXRel + $centerX;
                        $srcImageY = $centerY - $newYRel;
                    }
                    
                    $pixelColor = imagecolorat  ($image, $srcImageX, $srcImageY); // get source pixel color
                    
                    imagesetpixel ($dstImage, $dstImageX, $dstImageY, $pixelColor); // write destination pixel
                }
            }
        }
        
        return $dstImage;
    }

<?php

// auto rotates an image file based on exif data from camera
// if destination file is specified then it saves file there, otherwise it will display it to user
// note that images already at normal orientation are skipped (when exif data Orientation = 1)

if(!function_exists("gd_auto_rotate")){
    function gd_auto_rotate($original_file, $destination_file=NULL){
        
        $original_extension = strtolower(pathinfo($original_file, PATHINFO_EXTENSION));
        if(isset($destination_file) and $destination_file!=''){
            $destination_extension = strtolower(pathinfo($destination_file, PATHINFO_EXTENSION));
        }
        
        // try to auto-rotate image by gd if needed (before editing it)
        // by imagemagik it has an easy option
        if(function_exists("exif_read_data")){
            
            $exif_data = exif_read_data($original_file);
            $exif_orientation = $exif_data['Orientation'];
            
            // value 1 = normal ?! skip it ?!
            
            if($exif_orientation=='3'  or $exif_orientation=='6' or $exif_orientation=='8'){
                
                $new_angle[3] = 180;
                $new_angle[6] = -90;
                $new_angle[8] = 90;
                
                // load the image
                if($original_extension == "jpg" or $original_extension == "jpeg"){
                    $original_image = imagecreatefromjpeg($original_file);
                }
                if($original_extension == "gif"){
                    $original_image = imagecreatefromgif($original_file);
                }
                if($original_extension == "png"){
                    $original_image = imagecreatefrompng($original_file);
                }
                
                $rotated_image = imagerotate($original_image, $new_angle[$exif_orientation], 0);
                
                // if no destination file is set, then show the image
                if(!$destination_file){
                    header('Content-type: image/jpeg');
                    imagejpeg($rotated_image, NULL, 100);
                }
                        
                // save the smaller image FILE if destination file given
                if($destination_extension == "jpg" or $destination_extension=="jpeg"){
                    imagejpeg($rotated_image, $destination_file,100);
                }
                if($destination_extension == "gif"){
                    imagegif($rotated_image, $destination_file);
                }
                if($destination_extension == "png"){
                    imagepng($rotated_image, $destination_file,9);
                }
                
                imagedestroy($original_image);
                imagedestroy($rotated_image);
            
            }
        }
    }
}

?>

<?php
$urlImage = "images/image.png";
$urlNewImage = "images/imageNew.png";
    
$src_img = imagecreatefrompng($urlImage);
$originalX = imagesx($src_img);
$originalY = imagesy($src_img);
$dst_img = imagecreatetruecolor(640,480);

    
$imageRotate = imagerotate($src_img,20,-1);
        
imagepng($imageRotate,$urlNewImage);
imagedestroy($dst_img); 
imagedestroy($src_img);
?>

There is a slight mistake in the previous post by Kae Cyphet.

When setting the pixels of the new image the co-ordinates have to occasionally use the width ($wid) and height ($hei). These values must be reduced by 1 as the co-ordinates start at 0 (Not 1) so they only go up to $wid - 1 and $hei - 1 (Not $wid and $hei).

Here's an example of the first function to show this:

<?php
function rotate_right90($im)
{
 $wid = imagesx($im);
 $hei = imagesy($im);
 $im2 = imagecreatetruecolor($hei,$wid);

 for($i = 0;$i < $wid; $i++)
 {
  for($j = 0;$j < $hei; $j++)
  {
   $ref = imagecolorat($im,$i,$j);
   imagesetpixel($im2,($hei - 1) - $j,$i,$ref);
  }
 }
 return $im2;
}
?>

Other than that very useful functions!