This script creates transparency for a unspecific RBG color for an already created PNG image.  It also includes script of overlay text that does not get ruined in the process. 

<?php

header("Content-type: image/png");
$image = imagecreatetruecolor(250, 250);
$string = $_GET['text'];
$im     = imagecreatefrompng("dynIcon.png");
$img = imagecreatetruecolor(16,16);
$orange = imagecolorallocate($im, 220, 210, 60);
$bg_color = imagecolorat($im,1,1);
$px     = (imagesx($im) - 3 * strlen($string)) / 2;
imagecolortransparent($im, $bg_color);
imagestring($im, 3, $px, 5, $string, $orange);
imagepng($im);
imagedestroy($im);

?> 

Use $bg_color = imagecolorat($im,1,1); for instance, if you made the transparent color in photoshop by clearing out all the color leaving you with the checkered background showing you it's empty.

I've made a very simple script that will retain transparency of images especially when resizing.

NOTE: Transparency is only supported on GIF and PNG files.

Parameters:

$new_image = image resource identifier such as returned by imagecreatetruecolor(). must be passed by reference
$image_source = image resource identifier returned by imagecreatefromjpeg, imagecreatefromgif and imagecreatefrompng. must be passed by reference

<?php
function setTransparency($new_image,$image_source)
    {
        
            $transparencyIndex = imagecolortransparent($image_source);
            $transparencyColor = array('red' => 255, 'green' => 255, 'blue' => 255);
             
            if ($transparencyIndex >= 0) {
                $transparencyColor    = imagecolorsforindex($image_source, $transparencyIndex);    
            }
            
            $transparencyIndex    = imagecolorallocate($new_image, $transparencyColor['red'], $transparencyColor['green'], $transparencyColor['blue']);
            imagefill($new_image, 0, 0, $transparencyIndex);
             imagecolortransparent($new_image, $transparencyIndex);
        
    } 
?>


Sample Usage: (resizing)

<?php
$image_source = imagecreatefrompng('test.png');
$new_image = imagecreatetruecolor($width, $height);
setTransparency($new_image,$image_source);
imagecopyresampled($new_image, $image_source, 0, 0, 0, 0, $new_width, $new_height, $old_width, $old_height);
?>

Pay attention, that some GIF images may not include a transparent color. A good example of forced transparency in resized GIF image was given by markglibres at yahoo dot com 29-Mar-2009 02:48. But sometimes the transparent color in GIF images can be not set. The problem is, that the color you force to be transparent can be used in the original GIF as opaque and you will loose that color in resized image. The solution is not to use some default transparent color and to leave the resized image without transparent color (the same as original GIF). I used (nearly) the following code to make resized GIF images trnsparent  only when the transparency is needed:

<?php
/* ... */

$img = ImageCreateFromGIF($f); /* create image from existing GIF: original image file name $f you may take from where you need */
$transparent_index = ImageColorTransparent($img); /* gives the index of current transparent color or -1 */
if($transparent_index!=(-1)) $transparent_color = ImageColorsForIndex($img,$transparent_index);

/* ... */

$img_resized = ImageCreateTrueColor( $nw, $nh ); /* (the new width $nw and height $nh must be defined before) */
if(!empty($transparent_color)) /* simple check to find wether transparent color was set or not */
{
    $transparent_new = ImageColorAllocate( $img_resized, $transparent_color['red'], $transparent_color['green'], $transparent_color['blue'] );
    $transparent_new_index = ImageColorTransparent( $img_resized, $transparent_new );
    ImageFill( $img_resized, 0,0, $transparent_new_index ); /* don't forget to fill the new image with the transparent color */
}
list($w,$h) = GetImageSize($f); /* defines the original width $w and height $h */
if( ImageCopyResized( $img_resized, $img, 0,0, 0,0, $nw,$nh, $w,$h ) ) /* resized copying and replacing the original image */
{
    ImageDestroy($img);
    $img = $img_resized;
}

/* ... */

header('Content-Type: image/gif');
ImageGIF($img);
ImageDestroy($img);
?>

P.S.
If there some errors, I hope you could understood the idea.

This function has a very strange behaviour with GD version > 2. It returns count of colors instead of -1 (as noted) if cannot find transparent color. Be carefull!

How to center your text both vertically and horizontally in a GD image:

<?php
//get box size
$box = imagettfbbox($titlesize, 0, $titlefont, $title);
        
//Find out the width and height of the text box
$textW = $box[2] - $box[0];
$textH = $box[5] - $box[3];
        
// Calculate the positions
$positionLeft = ($width - $textW)/2;
$positionTop = (($height - $textH)/2);
        
// Add some text
if($align=="center"){    
imagettftext($pic, size, angle, $positionLeft, $positionTop, colour, font, message);
?>

To resize transparent PNG (if image is transparent & ImageColorTransparent() returns -1):
1) create a new image with the new sizes
2) make the new image all transparent
3) turn off the alpha blending for the new image (to keep the alpha channel when copy data)
4) do copyresampled or copyresized into new image

PHP code:
// 1
$im2 = ImageCreateTrueColor($w, $h);
// 2
ImageColorTransparent($im2, ImageColorAllocate($im2, 0, 0, 0));
// 3
ImageAlphaBlending($im2, false);
// 4
ImageCopyResampled($im2, $im, 0, 0, 0, 0, $w, $h, ImageSX($im), ImageSY($im));

Only one color may be transparent in one image.  The last call to imagecolortransparent will be the color that is set to transparent.

I am processing button images that have a slightly different fill color than the background color outside the border of the button. I was hoping that I could just make both of those colors transparent and solve the problem.

Hope this tidbit of info will save you some time.

I found out the hard way that for two png's with alpha transparency to correctly stack you need to explicitly re-enable alpha blending.

$img=imagecreatetruecolor($dx,$dy);
$img1=imagecreatefrompng($png1); //first layer
$img2=imagecreatefrompng($png2); //second layer (smaller image)

imagealphablending($img, false);
$transparent = imagecolorallocatealpha($img, 0, 0, 0, 127);
imagefill($img, 0, 0, $transparent);
imagesavealpha($img,true);
imagealphablending($img, true); //removing this causes the second layer's transparency to go trough the 1st layer erasing it (the image >is< transparent there ... as is the 2nd layer ... but not the 1st so it should not be transparent)

imagecopyresampled($img,$img1,
imagecopyresampled($img,$img2,

After much devious mindbending, I have found a way to test any GIF for presence of background transparency. This ability is essential for my application which uploads any of GIF, JPEG or PNG and simultaneously creates a thumbnail of identical image type and identical filename (full size and thumbnail versions being stored in different folders).
After uploading and moving the image in the usual way, a switch($image_type) statement ensures that the optimum code is used to generate the thumbnail; regardless of image type.
The problem with the GIF type is that those with transparent backgrounds need to be treated differently to those without. When I don't detect GIF transparency, I either end up with all transparent GIF's having black backgrounds, or all GIF's get converted to transparent background - even if they weren't transparent in the original.
But how to detect transparency in the original? It finally occurred to me that I could test for transparency programmatically by overlaying a copy of the original image over an all-black image, record the color value at particular pixel locations and then repeat the process by overlaying a copy of the original image over an all-white image, recording the color values at identical pixel locations and comparing these with the first set of values.
If the two sets of values correlate exactly, and if sensible sampling points are used, the image can be treated as non-transparent. If the two sets of values show differences, the image should be treated as having a transparent background.

This is my 'perfect' (i use that word lightly) thumbnail generation script, switch '$transparency' to true to have it do its best to handle transparency in gifs and pngs. this code is built off of comments and advice of everyone else here, and i do not deserve full credit. So far this handles every error i can throw at it.
<?php
function createthumb($name, $newname, $new_w, $new_h, $border=false, $transparency=true, $base64=false) {
    if(file_exists($newname))
        @unlink($newname);
    if(!file_exists($name))
        return false;
    $arr = split("\.",$name);
    $ext = $arr[count($arr)-1];

    if($ext=="jpeg" || $ext=="jpg"){
        $img = @imagecreatefromjpeg($name);
    } elseif($ext=="png"){
        $img = @imagecreatefrompng($name);
    } elseif($ext=="gif") {
        $img = @imagecreatefromgif($name);
    }
    if(!$img)
        return false;
    $old_x = imageSX($img);
    $old_y = imageSY($img);
    if($old_x < $new_w && $old_y < $new_h) {
        $thumb_w = $old_x;
        $thumb_h = $old_y;
    } elseif ($old_x > $old_y) {
        $thumb_w = $new_w;
        $thumb_h = floor(($old_y*($new_h/$old_x)));
    } elseif ($old_x < $old_y) {
        $thumb_w = floor($old_x*($new_w/$old_y));
        $thumb_h = $new_h;
    } elseif ($old_x == $old_y) {
        $thumb_w = $new_w;
        $thumb_h = $new_h;
    }
    $thumb_w = ($thumb_w<1) ? 1 : $thumb_w;
    $thumb_h = ($thumb_h<1) ? 1 : $thumb_h;
    $new_img = ImageCreateTrueColor($thumb_w, $thumb_h);
    
    if($transparency) {
        if($ext=="png") {
            imagealphablending($new_img, false);
            $colorTransparent = imagecolorallocatealpha($new_img, 0, 0, 0, 127);
            imagefill($new_img, 0, 0, $colorTransparent);
            imagesavealpha($new_img, true);
        } elseif($ext=="gif") {
            $trnprt_indx = imagecolortransparent($img);
            if ($trnprt_indx >= 0) {
                //its transparent
                $trnprt_color = imagecolorsforindex($img, $trnprt_indx);
                $trnprt_indx = imagecolorallocate($new_img, $trnprt_color['red'], $trnprt_color['green'], $trnprt_color['blue']);
                imagefill($new_img, 0, 0, $trnprt_indx);
                imagecolortransparent($new_img, $trnprt_indx);
            }
        }
    } else {
        Imagefill($new_img, 0, 0, imagecolorallocate($new_img, 255, 255, 255));
    }
    
    imagecopyresampled($new_img, $img, 0,0,0,0, $thumb_w, $thumb_h, $old_x, $old_y); 
    if($border) {
        $black = imagecolorallocate($new_img, 0, 0, 0);
        imagerectangle($new_img,0,0, $thumb_w, $thumb_h, $black);
    }
    if($base64) {
        ob_start();
        imagepng($new_img);
        $img = ob_get_contents();
        ob_end_clean();
        $return = base64_encode($img);
    } else {
        if($ext=="jpeg" || $ext=="jpg"){
            imagejpeg($new_img, $newname);
            $return = true;
        } elseif($ext=="png"){
            imagepng($new_img, $newname);
            $return = true;
        } elseif($ext=="gif") {
            imagegif($new_img, $newname);
            $return = true;
        }
    }
    imagedestroy($new_img); 
    imagedestroy($img); 
    return $return;
}
//example useage
createthumb("img.gif", "tn_img.gif", 64,64,true, true, false);
?>

a simple way to check png with alpha channel reading IHDR PNG HEADER

$readPng =    fopen    ($argSourceImagePath, "rb");
$readAlp =    fread    ($readPng, 52);
            fclose    ($readPng);

if(substr(bin2hex($readAlp),50,2) == "04" || substr(bin2hex($readAlp),50,2) == "06")
echo("Png has alpha");

in reference to webmaster at webnetwizard dotco dotuk who had a rather complicated method of determining if a GIF had any transparency set ...

imagecolortransparent will return -1 if no transparency is found.

eg:

$transColorIndexMain = imageColorTransparent($mainImgObj);
if ($transColorIndexMain >= 0 ) {
       # GIF has transparency ... ;
}

Current PHP Version: 4.4.4
[GD Version] => bundled (2.0.28 compatible)

To resize or copy image (gif [with gd>=2.0.28] or png) with transparency.
-Take current transparency color
-create a new image with palette like old one and fill new image with transparent color
-set transparent color
-copy resized image

$colorTransparent = imagecolortransparent($im);
$im2 = imagecreate($new_w,$new_h);
imagepalettecopy($im2,$im);
imagefill($im2,0,0,$colorTransparent);
imagecolortransparent($im2, $colorTransparent);
imagecopyresized($im2,$im,0,0,0,0,$new_w,$new_h,$imsx,$imsy);

Hi!
I'm using GDLib 1.6.3 so far I know.
You can have transparency on any defined color on any image type when using the imagecolorclosest($im, $R, $G, $B) function instead of imagecolorallocate() or imagecolorexact().

If you are looking for a blank PNG, you don't need to generate it every time. Just define this constant:

define("BLANK_PNG",     "iVBORw0KGgoAAAANSUhEUgAAACgAAAAoCAYAAACM/rhtAAAABGdBTUEAAK/INwWK6QAAABl0RVh0U29m".
                        "dHdhcmUAQWRvYmUgSW1hZ2VSZWFkeXHJZTwAAADqSURBVHjaYvz//z/DYAYAAcTEMMgBQAANegcCBNCg".
                        "dyBAAA16BwIE0KB3IEAADXoHAgTQoHcgQAANegcCBNCgdyBAAA16BwIE0KB3IEAADXoHAgTQoHcgQAAN".
                        "egcCBNCgdyBAAA16BwIE0KB3IEAADXoHAgTQoHcgQAANegcCBNCgdyBAAA16BwIE0KB3IEAADXoHAgTQ".
                        "oHcgQAANegcCBNCgdyBAAA16BwIE0KB3IEAADXoHAgTQoHcgQAANegcCBNCgdyBAAA16BwIE0KB3IEAA".
                        "DXoHAgTQoHcgQAANegcCBNCgdyBAgAEAMpcDTTQWJVEAAAAASUVORK5CYII=");

A transparent background with text doesn't seem to work very well because of the antialiasing . But I tried the following kluge and it turned out very well:

<?php
   $im = imagecreatetruecolor(100,20);
   $almostblack = imagecolorallocate($im,254,254,254);
   imagefill($im,0,0,$almostblack);
   $black = imagecolorallocate($im,0,0,0);
   imagecolortransparent($im,$almostblack);
   //... set x and y..
   imagettftext($im,8,0,$x,$y,$black,"calibri.ttf",$txt);
?>

I would like to share with others how to make PNG image transparent. I have tried almost all examples described on that page but none of them work for me. Finally I found the solution, but there is a bit cheating :) but who cares if it works?

So when I tried to allocate white (as an example) color as a transparent, it worked randomly (lets say 1 in 10 examples). That in turn pushed me to the idea that the problem is that white color is already exist in color palette. So if I try to add another white color with DIFFERENT index as a transparent, that will cause error.

So I found the solution, instead of adding new index, I searched for the white color’s index in the color palette and made it defined as transparent :) The best idea is to use not a white but left bottom corner as transparent color (I guess that s standard for some other software languages).

<?php
$index = imagecolorexact($this->image, 255, 255, 255);
imagecolortransparent($this->image, $index);
?>

All you need is to add that two lines before outputting your result.

This might be of importance to newbies...If you are trying to create GIF's with transparencies, stick to the 216 colour pallet for all colours on the image... or transparencies wont work...at the very least yo will get dithering...

Well 8-bit gif and png files can have multiple transparent colors, so detecting the transparent color by manually looking into the file or using imagecolortransparent and using the detected rgb color as the background color when imagecopy'ing (or other image copying functions) on a true color image will not work. The only means of getting the transparent pixels to appear is to copy the pixel colors one by one to a new true color image. Here's some code i wrote to do this.

$original_image = @imagecreatefromgif( $original_image_file );

$original_image_width = imagesx( $original_image );
$original_image_height = imagesy( $original_image );

if ( !imageistruecolor($original_image) ) {
   # detects if its non-true color image
   if ( imagecolortransparent($original_image) >= 0 ) {
      # detects if any of the color palette has been set to transparent
      $truecolor = imagecreatetruecolor( $original_image_width, $original_image_height );
      for ($x = 0; $x < $original_image_width; $x++) {
         for ($y = 0; $y < $original_image_height; $y++) {
            $color_index = ImageColorAt($original_image, $x, $y);
            if ( !$color_palette[$color_index] ) {
               $rgb = imagecolorsforindex($original_image, $color_index);
               $color_to_set = imagecolorallocate($truecolor, $rgb['red'], $rgb['green'], $rgb['blue']);
               $color_palette[$color_index] = $color_to_set;
            } else {
               $color_to_set = $color_palette[$color_index];
            }
            imagesetpixel($truecolor, $x, $y, $color_to_set);
         }
      }
      imagedestroy($original_image);
      $original_image = $truecolor;
   }
}

Sorry, below should be:

<?php
$is_alpha = ord (file_get_contents ($file_path, false, null, 25, 1)) & 4;
?>

much better :)
Both types 4 (greyscale transparent png), and 6 (colour transparent png) have bit 4 set, so there's no need to bitwise twice and there's no need for == as the return value will be 4 or 0, which is easily interpreted as true or false elsewhere in your code.

note: file_get_contents isn't reading the whole file in this instance, just 1 byte on its own and that's it, so you can be assured this is fast and safe.

@wesley gunn:
Thank you very, very much for the code to determine if a PNG is 24 or 32-bit :)
Here's an improved version in one line without using a file-handle

<?php
$is_alpha = ((ord (
    file_get_contents ($filename, false, null, 25, 1)
) & 6) & 4) == 4;
?>

Gives true for a PNG with alpha, false otherwise.
This cannot detect 8-bit PNGs with transparency, but you could do that by searching the file contents for the presence of "PLTE" *AND* "tRNS".

On some versions you must set the index number of that color in color pallete rather than the color! So if you allocate color with imagecolorallocate, that this color gets 0 index on color pallete so than you must call: imagecolortransparent ( resource $image , 0 ) to make the image transparent!

When you use palette images (created with imagecreate()), the first color allocated is the background color. This color cannot be used for transparency. So if you want to make the background transparent, first allocate a dummy background color, then allocate the real background color and declare this is as transparent.

This script should take an animated GIF whit a white background transparancy, put some white=transparant text on it, and can be used in any page using the filename and the normal image tags.

Everything works well, exept the animation. The GIF animation stands still. Don't know how to keep the animation  animated. Is it possible in PHP?

<?php

$TextFile = "teller.txt";
$Count = trim(file_get_contents($TextFile));
$FP = fopen($TextFile, "r");
$Count=fgets($FP, 4096);
fclose ($FP);
settype($Count, "integer");
$Count++;
if ($FP = fopen ($TextFile, "w")){
fwrite ($FP, $Count);
fclose ($FP);
}

$image = "blad.gif";
$im    = imagecreatefromgif($image);
$white = imageColorAllocate ($im, 255, 255, 255);
$trans = imagecolortransparent($im,$white);
$hit = "$Count";
$ip = $_SERVER["REMOTE_ADDR"];

ImageString($im, 2, 30, 60,  " je bent bezoeker ", $white);
ImageString($im, 3, 20, 80, " $ip.$hit", $trans);

header("Content-Type: image/gif");
Imagegif($im,'',100);
ImageDestroy ($im);

?>

In fact this function can be used for two purposes:

A.) to work with transparency while dealing with images in php (see watermark example)

B.) to create a partially transparent picture

For A.) one can work with truecolor images with some regulations, but B.) can only be used with palette-based images (= indexed = created with imagecreate not imagecreatetruecolor) and formats that support transparency (png, gif).

For instance if you want to cut out a color out of a give picture that is in truecolor, use the following method to first convert it to palette-based image, assign the transparency and give it to the browser as png:

<?
$img = imagecreatefromjpeg('test.jpg');
imagetruecolortopalette($img, false, 256); // convert

$white = imagecolorresolve($img, 255, 255, 255); // resolve given palette entry
imagecolortransparent($img, $white);

header("Content-type: image/png");
imagepng($img);
?>

i had problems with the example by sandhawk at spies dot com because my png overlay, and the jpeg canvas were using different color depths, so, this function corrects this:

[code]
function WatermarkImage($CanvasImage, $WatermarkImage /* MUST BE PHG */, $Opacity=10, $Quality=75)
{
    // create true color canvas image:
    $canvas_src = imagecreatefromjpeg($CanvasImage);
    $canvas_w = ImageSX($canvas_src);
    $canvas_h = ImageSY($canvas_src);
    $canvas_img = imagecreatetruecolor($canvas_w, $canvas_h);
    imagecopy($canvas_img, $canvas_src, 0,0,0,0, $canvas_w, $canvas_h);
    imagedestroy($canvas_src);    // no longer needed

    // create true color overlay image:
    $overlay_src = imagecreatefrompng($WatermarkImage);
    $overlay_w = ImageSX($overlay_src);
    $overlay_h = ImageSY($overlay_src);
    $overlay_img = imagecreatetruecolor($overlay_w, $overlay_h);
    imagecopy($overlay_img, $overlay_src, 0,0,0,0, $overlay_w, $overlay_h);
    imagedestroy($overlay_src);    // no longer needed

    // setup transparent color (pick one):
    $black   = imagecolorallocate($overlay_img, 0x00, 0x00, 0x00);
    $white   = imagecolorallocate($overlay_img, 0xFF, 0xFF, 0xFF);
    $magenta = imagecolorallocate($overlay_img, 0xFF, 0x00, 0xFF);    
    // and use it here:
    imagecolortransparent($overlay_img, $white);

    // copy and merge the overlay image and the canvas image:
    imagecopymerge($canvas_img, $overlay_img, 0,0,0,0, $overlay_w, $overlay_h, $Opacity);

    // output:
    header("Content-type: image/jpeg");
    imagejpeg($canvas_img, '', $Quality);

    imagedestroy($overlay_img);
    imagedestroy($canvas_img);
}

// call function with opcity set to 50% and 95% quality
WatermarkImage("canvas.jpg", "overlay.png", 50, 95);

[/code]

ok some works...

i had a test-sytem with GDLib, phpinfo shows me this version:
"GD Version  2.0 or higher" so far no porblems with: imageColorTransparent

Then we had to copy ouer codes to another sever on this server phpinfo shots me version:
"GD Version  bundled (2.0.12 compatible)"

The jpg was wrong and nothing was transparent.
The point is, you have to use: imagecopymerge

Don't if its true on any system just work on 
SUSE 8.2
PHP 4.3.2

I have found that ImageColorTransparent() works on true color images if and only if the transparent color is black.
(PHP 4.2.3/GD 2.0)