As noted below, getimagesize will download the entire image before it checks for the requested information. This is extremely slow on large images that are accessed remotely. Since the width/height is in the first few bytes of the file, there is no need to download the entire file. I wrote a function to get the size of a JPEG by streaming bytes until the proper data is found to report the width and height:

<?php
// Retrieve JPEG width and height without downloading/reading entire image.
function getjpegsize($img_loc) {
    $handle = fopen($img_loc, "rb") or die("Invalid file stream.");
    $new_block = NULL;
    if(!feof($handle)) {
        $new_block = fread($handle, 32);
        $i = 0;
        if($new_block[$i]=="\xFF" && $new_block[$i+1]=="\xD8" && $new_block[$i+2]=="\xFF" && $new_block[$i+3]=="\xE0") {
            $i += 4;
            if($new_block[$i+2]=="\x4A" && $new_block[$i+3]=="\x46" && $new_block[$i+4]=="\x49" && $new_block[$i+5]=="\x46" && $new_block[$i+6]=="\x00") {
                // Read block size and skip ahead to begin cycling through blocks in search of SOF marker
                $block_size = unpack("H*", $new_block[$i] . $new_block[$i+1]);
                $block_size = hexdec($block_size[1]);
                while(!feof($handle)) {
                    $i += $block_size;
                    $new_block .= fread($handle, $block_size);
                    if($new_block[$i]=="\xFF") {
                        // New block detected, check for SOF marker
                        $sof_marker = array("\xC0", "\xC1", "\xC2", "\xC3", "\xC5", "\xC6", "\xC7", "\xC8", "\xC9", "\xCA", "\xCB", "\xCD", "\xCE", "\xCF");
                        if(in_array($new_block[$i+1], $sof_marker)) {
                            // SOF marker detected. Width and height information is contained in bytes 4-7 after this byte.
                            $size_data = $new_block[$i+2] . $new_block[$i+3] . $new_block[$i+4] . $new_block[$i+5] . $new_block[$i+6] . $new_block[$i+7] . $new_block[$i+8];
                            $unpacked = unpack("H*", $size_data);
                            $unpacked = $unpacked[1];
                            $height = hexdec($unpacked[6] . $unpacked[7] . $unpacked[8] . $unpacked[9]);
                            $width = hexdec($unpacked[10] . $unpacked[11] . $unpacked[12] . $unpacked[13]);
                            return array($width, $height);
                        } else {
                            // Skip block marker and read block size
                            $i += 2;
                            $block_size = unpack("H*", $new_block[$i] . $new_block[$i+1]);
                            $block_size = hexdec($block_size[1]);
                        }
                    } else {
                        return FALSE;
                    }
                }
            }
        }
    }
    return FALSE;
}
?>

There's a code snippet for getting JPEG image dimensions by getting only first few bytes of the file, but it doesn't work for PNG files, so I wrote one. It will download only the first 24 bytes instead of the whole image, and thus being much faster than getimagesize() and it will save bandwidth at the same time:

<?php
// Retrieve PNG width and height without downloading/reading entire image.
function getpngsize( $img_loc ) {
    $handle = fopen( $img_loc, "rb" ) or die( "Invalid file stream." );

    if ( ! feof( $handle ) ) {
        $new_block = fread( $handle, 24 );
        if ( $new_block[0] == "\x89" &&
            $new_block[1] == "\x50" &&
            $new_block[2] == "\x4E" &&
            $new_block[3] == "\x47" &&
            $new_block[4] == "\x0D" &&
            $new_block[5] == "\x0A" &&
            $new_block[6] == "\x1A" &&
            $new_block[7] == "\x0A" ) {
                if ( $new_block[12] . $new_block[13] . $new_block[14] . $new_block[15] === "\x49\x48\x44\x52" ) {
                    $width  = unpack( 'H*', $new_block[16] . $new_block[17] . $new_block[18] . $new_block[19] );
                    $width  = hexdec( $width[1] );
                    $height = unpack( 'H*', $new_block[20] . $new_block[21] . $new_block[22] . $new_block[23] );
                    $height  = hexdec( $height[1] );

                    return array( $width, $height );
                }
            }
        }

    return false;
}
?>

If you want to "convert" value returned by "getimagesize()" as index "2" into something more human-readable, you may consider using a function like this one:

    $imageTypeArray = array
    (
        0=>'UNKNOWN',
        1=>'GIF',
        2=>'JPEG',
        3=>'PNG',
        4=>'SWF',
        5=>'PSD',
        6=>'BMP',
        7=>'TIFF_II',
        8=>'TIFF_MM',
        9=>'JPC',
        10=>'JP2',
        11=>'JPX',
        12=>'JB2',
        13=>'SWC',
        14=>'IFF',
        15=>'WBMP',
        16=>'XBM',
        17=>'ICO',
        18=>'COUNT'  
    );
    
    $size = getimagesize($filename);
    
    $size[2] = $imageTypeArray[$size[2]];

Or something similar.

Note that, if you're going to be a good programmer and use named constatnts (IMAGETYPE_JPEG) rather than their values (2), you want to use the IMAGETYPE variants - IMAGETYPE_JPEG, IMAGETYPE GIF, IMAGETYPE_PNG, etc.  For some reason, somebody made a horrible decision, and IMG_PNG is actually 4 in my version of PHP, while IMAGETYPE_PNG is 3.  It took me a while to figure out why comparing the type against IMG_PNG was failing...

Note: getimage size doesn't attempt to validate image file formats

It is possible for malformed GIF images to contain PHP and still have valid dimensions.

Programmers need to ensure such images are validated by other tools, or never treated as PHP or other executable types (enforcing appropriate extensions, avoiding user controlled renaming, restricting uploaded images to areas of the website where PHP is not enabled).

http://ha.ckers.org/blog/20070604/passing-malicious-php-through-getimagesize/

It's always good to check out an image's dimensions while attempting to upload to your server or database...especially if it's going to be displayed on a page that doesn't accomodate images beyond a particular size.

<?php

$tmpName = $_FILES['userfile']['tmp_name'];
        
list($width, $height, $type, $attr) = getimagesize($tmpName);

if($width>275 || $height>275)
{
die("exceeded image dimension limits.");
}

?>

Here is the function which determines whether the PNG image contains alpha or not:

<?php
function is_alpha_png($fn){
  return (ord(@file_get_contents($fn, NULL, NULL, 25, 1)) == 6);
}
?>

The color type of PNG image is stored at byte offset 25. Possible values of that 25'th byte is:
 * 0 - greyscale
 * 2 - RGB
 * 3 - RGB with palette
 * 4 - greyscale + alpha
 * 6 - RGB + alpha

Seems the various ways people are trying to proportionaly scale an image, up or down, could be more straight forward if one remembers ones algebra.

The formula is, y = mx, where m is the slope of the line. This is the ratio of y:x or m = y/x.

So if...

// max values for x and y
$y_max = 600;
$x_max = 800;

// image size
$y1 = 2000;
$x1 = 3000;

// use width for scaling
if ($x1 > $x_max)
{
    // find slope 
    $m = $y1/$x1;
    // set x side to max
    $x2 = $x_max;
    // set y side to a proportional size
    $y2 = $m * $x1;
}

The new image proportionally scaled will be x2 = 800, y2 = 533 (rounded).

To do it from the y side, simply reverse the x's and y's.

The list of defined IMAGETYPE_ constants is on the manual page for exif_imagetype:

http://www.php.net/manual/en/function.exif-imagetype.php

Could be useful (didn´t know where to post it):

function getImageErrors( $filename, $type = "", $minWidth = 0, $minHeight = 0, $maxWidth = 0, $maxHeight = 0, $maxFileSize = 0 )
{
    $errors = array();
    if ( file_exists( $filename ) )
    {
        $ending = substr( $filename, strpos( $filename, "." ) );
        if ( is_array( $type ) )
        {
            $isTypeOf = false;
            foreach( $type as $eachtype )
            {
                if ( $ending == $eachtype )
                {
                    $isTypeOf = true;
                }
            }
            if ( ! $isTypeOf )
            {
                $errors[ 'type' ] = $ending;
            }
        }
        elseif ( $type != "" )
        {
            if ( $ending != $type )
            {
                $errors[ 'type' ] = $ending;
            }
        }
        $size = getimagesize( $filename );
        if ( $size[ 0 ] < $minWidth )
        {
            $errors[ 'minWidth' ] = $size[ 0 ];
        }
        if ( $size[ 1 ] < $minHeight )
        {
            $errors[ 'minHeight' ] = $size[ 1 ];
        }
        if ( ( $maxWidth > $minWidth ) && ( $size[ 0 ] > $maxWidth ) )
        {
            $errors[ 'maxWidth' ] = $size[ 0 ];
        }
        if ( ( $maxHeight > $minHeight ) && ( $size[ 1 ] > $maxHeight ) )
        {
            $errors[ 'maxHeight' ] = $size[ 1 ];
        }
        if ( ( $maxFileSize > 0 ) && ( filesize( $filename ) > $maxFileSize ) )
        {
            $errors[ 'maxFileSize' ] = filesize( $filename );
        }
    }
    else
    {
        $errors[ 'filename' ] = "not existing";
    }
    return ( count( $errors ) > 0 ? $errors : null );
}

Well, I am making a script which will resize the image when uploaded, however, i am making a multi-uploader, so i came across with a problem: an efficient way of getting a pictures height and width and storing them in an array to resize later. This is what i came up with:

<?php
$links = array("test1.jpg", "test2.png");
$sizearray = array();
$count = count($links);
for($i = 0; $i < $count; $i++) {
    $size = getimagesize($links[$i]);
    list($width, $height) = $size;
    $sizearray[$links[$i]] = array("width" => $width, "height" => $height);
}
print_r($sizearray);
// which will print out: Array ( [test1.jpg] => Array ( [width] => 300 [height] => 400 ) [test2.png] => Array ( [width] => 680 [height] => 100 ) )
?>

I'm sorry for they other scripts, but I made one mistake about the image resizing... here is a working script !
<?
    // Some configuration variables !
    $maxWidth = 90;
    $maxHeight = 90;
    $maxCols = 8;
    $webDir = "https://localhost/images/";
    $localDir = $_SERVER['DOCUMENT_ROOT']."/images/";

    $AutorisedImageType = array ("jpg", "jpeg", "gif", "png");
?>

<center>
<table border='1' cellspacing='5' cellpadding='5' style="border-collapse:collapse; border-style: dotted">
<tr>
   <?
   // Open localDir
   $dh = opendir($localDir);
   while (false !== ($filename = readdir($dh))) {
       $filesArray[] = $filename;
   }

   // Display and resize
   foreach ($filesArray as $images) {
   
       $ext = substr($images, strpos($images, ".")+1, strlen($images));
       
       if( in_array($ext, $AutorisedImageType) ) {

           list($width, $height, $type, $attr) = @getimagesize( $localDir.$images );

            $xRatio = $maxWidth / $width; 
            $yRatio = $maxHeight / $height; 
            
            if ( ($width <= $maxWidth) && ($height <= $maxHeight) ) { 
              $newWidth = $width; 
              $newHeight = $height; 
            } 
            else if (($xRatio * $height) < $maxHeight) { 
              $newHeight = ceil($xRatio * $height); 
              $newWidth = $maxWidth; 
            } 
            else { 
              $newWidth = ceil($yRatio * $width); 
              $newHeight = $maxHeight; 
            } 
           
           if($i == $maxCols) {
               echo "</tr><tr>";
               $i = 0;
           }
           echo "<td align='center' valign='middle' width='$maxWidth' height='$maxHeight'><img src='".$webDir.$images."' width='$newWidth' height='$newHeight'></td>";
           $i++;
       }
   }
?>
</tr>
</table>
</center>

i made function img_resize($path,$tmp_name,$new_name,$new_width)
this could be useful.

<?php

$new_file = img_resize("./img/", "test.jpg","copy_test.jpg",300);
echo "<IMG src = '$new_file'>";

function img_resize($path,$tmp_name,$new_name,$new_width){
    if (!file_exists($path.$filename)){
        echo "file not found!";
        exit;
    }
    if (!is_writable($path)){
        echo "error:permission denied!";
        exit;
    }
    list($width, $height) = getimagesize($path . $tmp_name);
    $new_height = abs($new_width * $height / $width); 
    $image_p = imagecreatetruecolor($new_width, $new_height);
    $image = imagecreatefromjpeg($path . $tmp_name); 
    imagecopyresampled($image_p, $image, 0, 0, 0, 0,
                        $new_width, $new_height, $width, $height); 
    imagejpeg($image_p, $path . $new_name); 
    return $path.$new_name;
}

?>

Rather than making a lengthy function that essentially runs twice (once as width, once as height) I came up with a helpful function that uses variable variables to set a maximum height/width. Hope someone finds this helpful.

function scaleimage($location, $maxw=NULL, $maxh=NULL){
    $img = @getimagesize($location);
    if($img){
        $w = $img[0];
        $h = $img[1];

        $dim = array('w','h');
        foreach($dim AS $val){
            $max = "max{$val}";
            if(${$val} > ${$max} && ${$max}){
                $alt = ($val == 'w') ? 'h' : 'w';
                $ratio = ${$alt} / ${$val};
                ${$val} = ${$max};
                ${$alt} = ${$val} * $ratio;
            }
        }

        return("<img src='{$location}' alt='image' width='{$w}' height='{$h}' />");
    }
}

This function returns the width and height of a JPEG image from a string, allowing the dimensions of images stored in a database to be retrieved without writing them to the disk first, or using "imagecreatefromstring" which is very slow in comparison.

<?PHP
function getJPEGImageXY($data) {
        $soi = unpack('nmagic/nmarker', $data);
        if ($soi['magic'] != 0xFFD8) return false;
        $marker = $soi['marker'];
        $data   = substr($data, 4);
        $done   = false;

        while(1) {
                if (strlen($data) === 0) return false;
                switch($marker) {
                        case 0xFFC0:
                                $info = unpack('nlength/Cprecision/nY/nX', $data);
                                return array($info['X'], $info['Y']);
                                break;

                        default:
                                $info   = unpack('nlength', $data);
                                $data   = substr($data, $info['length']);
                                $info   = unpack('nmarker', $data);
                                $marker = $info['marker'];
                                $data   = substr($data, 2);
                                break;
                }
        }
}
?>

Doing this 10,000 times takes 0.43 seconds, compared with using imagecreatefromstring/imagesx/imagesy which takes around 1.52 seconds to do the same.

Do not use this instead of getimagesize when dealing with files, getimagesize is much faster coming in at 0.15 seconds.

Looking at the number of channels, you can detect if the image SUPPORTS transparecy

function image_supports_transparency(string $file_path): ?bool {
    $ret = \getimagesize($file_path);
    $image_type = $ret[2];
    $supports_transparency = null;

    if (isset($ret['channels'])) {
        $channels = $ret['channles'];

        if ($image_type === \IMAGETYPE_JPEG || $image_type === \IMAGETYPE_GIF) {
            $supports_transparency = false;
        } elseif ($image_type === \IMAGETYPE_PNG) {
            if ($channels === 2 || $channels === 4) {
                $supports_transparency = true;
            } elseif ($channels !== 1) {
                // PNG also supports transparency via palette, which only uses a single channel.
                $supports_transparency = false;
            }
        } elseif ($image_type === \IMAGETYPE_WEBP) {
            $supports_transparency = $channels === 4;
        } elseif ($mime === 'image/avif') {
            $supports_transparency = $channels === 4 || $channels == 2;
        } elseif ($image_type === \IMAGETYPE_BMP) {
            $supports_transparency = $channels === 4;
        }
    }

    return $supports_transparency;
}

I say supports because an image may have an alpha channel but not actually use it, you'll need to check all the image's pixels to detect that.
Also note that it's not always possible to detect the alpha channel with just getimagesize(); see for example png which can define the number of channles actually used in the palette chunk.

For some images, using getimagesize() without the second parameter will return the correct info, but when you add the second parameter it will return false. This is most likely a bug (and it has been reported as such), but meanwhile, if you encounter this problem, a workaround is to use exif_read_data().

I wanted to use getimagesize() on .SWF files stored in the database as blob data and couldn't find a simple solution, so I created my own.

I am releasing this code under the MIT license to save everyone some time:

<?php
/*
    ----------------------------------------------------------------------
    PHP Blob Data As File Stream v1.0 (C) 2012 Alex Yam <[email protected]>
    This code is released under the MIT License.
    ----------------------------------------------------------------------
    [Summary]

    A simple class for PHP functions to read and write blob data as a file
    using a stream wrapper.

    Particularly useful for running getimagesize() to get the width and
    height of .SWF Flash files that are stored in the database as blob data.

    Tested on PHP 5.3.10.

    ----------------------------------------------------------------------    
    [Usage Example]

    //Include
        include('./blob_data_as_file_stream.php');

    //Register the stream wrapper
        stream_wrapper_register("BlobDataAsFileStream", "blob_data_as_file_stream");

    //Fetch a .SWF file from the Adobe website and store it into a variable.
    //Replace this with your own fetch-swf-blob-data-from-database code.
        $swf_url = 'http://www.adobe.com/swf/software/flash/about/flashAbout_info_small.swf';
        $swf_blob_data = file_get_contents($swf_url);
    
    //Store $swf_blob_data to the data stream
        blob_data_as_file_stream::$blob_data_stream = $swf_blob_data;
    
    //Run getimagesize() on the data stream
        $swf_info = getimagesize('BlobDataAsFileStream://');
        var_dump($swf_info);

    ----------------------------------------------------------------------
    [Usage Output]

    array(5) {
      [0]=>
      int(159)
      [1]=>
      int(91)
      [2]=>
      int(13)
      [3]=>
      string(23) "width="159" height="91""
      ["mime"]=>
      string(29) "application/x-shockwave-flash"
    }

*/

class blob_data_as_file_stream {

    private static $blob_data_position = 0;
    public static $blob_data_stream = '';

    public static function stream_open($path,$mode,$options,&$opened_path){
        static::$blob_data_position = 0;
        return true;
    }

    public static function stream_seek($seek_offset,$seek_whence){
        $blob_data_length = strlen(static::$blob_data_stream);
        switch ($seek_whence) {
            case SEEK_SET:
                $new_blob_data_position = $seek_offset;
                break;
            case SEEK_CUR:
                $new_blob_data_position = static::$blob_data_position+$seek_offset;
                break;
            case SEEK_END:
                $new_blob_data_position = $blob_data_length+$seek_offset;
                break;
            default:
                return false;
        }
        if (($new_blob_data_position >= 0) AND ($new_blob_data_position <= $blob_data_length)){
            static::$blob_data_position = $new_blob_data_position;
            return true;
        }else{
            return false;
        }
    }

    public static function stream_tell(){
        return static::$blob_data_position;
    }

    public static function stream_read($read_buffer_size){
        $read_data = substr(static::$blob_data_stream,static::$blob_data_position,$read_buffer_size);
        static::$blob_data_position += strlen($read_data);
        return $read_data;
    }

    public static function stream_write($write_data){
        $write_data_length=strlen($write_data);
        static::$blob_data_stream = substr(static::$blob_data_stream,0,static::$blob_data_position).
            $write_data.substr(static::$blob_data_stream,static::$blob_data_position+=$write_data_length);
        return $write_data_length;
    }

    public static function stream_eof(){
        return static::$blob_data_position >= strlen(static::$blob_data_stream);
    }

}
?>

Returns a array with 4 elements.
The 0 index is the width of the image in pixels.
The 1 index is the height of the image in pixels.
The 2 index is a flag for the image type:

1 = GIF, 2 = JPG, 3 = PNG, 4 = SWF, 5 = PSD, 6 = BMP, 7 = TIFF(orden de bytes intel), 8 = TIFF(orden de bytes motorola), 9 = JPC, 10 = JP2, 11 = JPX, 12 = JB2, 13 = SWC, 14 = IFF, 15 = WBMP, 16 = XBM. 

The 3 index contains ' height="yyy" width="xxx" '

A simple piece of code i wrote to proportionally resize an image to a max height and width then display it

<?php
// Max height and width
$max_width = 100;
$max_height = 100;

// Path to your jpeg

$upfile '/path/to/file.jpg';
    Header("Content-type: image/jpeg");
    
    $size = GetImageSize($upfile); // Read the size
          $width = $size[0];
          $height = $size[1];
          
          // Proportionally resize the image to the
          // max sizes specified above
          
          $x_ratio = $max_width / $width;
          $y_ratio = $max_height / $height;

          if( ($width <= $max_width) && ($height <= $max_height) )
          {
               $tn_width = $width;
               $tn_height = $height;
          }
          elseif (($x_ratio * $height) < $max_height)
          {
               $tn_height = ceil($x_ratio * $height);
               $tn_width = $max_width;
          }
          else
          {
               $tn_width = ceil($y_ratio * $width);
               $tn_height = $max_height;
          }
     // Increase memory limit to support larger files
     
     ini_set('memory_limit', '32M');
     
     // Create the new image!
     $src = ImageCreateFromJpeg($upfile);
     $dst = ImageCreateTrueColor($tn_width, $tn_height);
     ImageCopyResized($dst, $src, 0, 0, 0, 0, $tn_width, $tn_height, $width, $height);
     ImageJpeg($dst);
// Destroy the images
ImageDestroy($src);
ImageDestroy($dst);
?>

How about this for cropping images...

<?php

$imgfile = "img.jpg";
$cropStartX = 300;
$cropStartY = 250;
$cropW   = 200;
$cropH   = 200;

// Create two images
$origimg = imagecreatefromjpeg($imgfile);
$cropimg = imagecreatetruecolor($cropW,$cropH);

// Get the original size
list($width, $height) = getimagesize($imgfile);

// Crop
imagecopyresized($cropimg, $origimg, 0, 0, $cropStartX, $cropStartY, $width, $height, $width, $height);

// TODO: write code to save new image
// or, just display it like this:
header("Content-type: image/jpeg");
imagejpeg($cropimg);

// destroy the images
imagedestroy($cropimg);
imagedestroy($origimg);

?>

Note that if you specify a remote file (via a URL) to check the size of, PHP will first download the remote file to your server.

If you're using this function to check the size of user provided image links, this could constitute a security risk.  A malicious user could potentially link to a very large image file and cause PHP to download it.  I do not know what, if any, file size limits are in place for the download.  But suppose the user provided a link to an image that was several gigabytes in size?

It would be nice if there were a way to limit the size of the download performed by this function.  Hopefully there is already a default with some sensible limits.

When validating images, allways check both, image type *AND* file extension!

Because most image types allow sections for comments or other irrelevant data. Those section can be used to infiltrate php code onto the server. If these files are stored as sent by the client, files with a ".php" extension can be executed and do tremendous harm.

Heres a easy way to scale images to the <td> that they are in
*this is broken up so anyone can understand it :)

<?
$imageinfo = getimagesize("images/picture.jpg");
          
$ix=$imageinfo[0];
$iy=$imageinfo[1];

$widthscale = $ix/175;  //<TD> WIDTH
$heightscale = $iy/175; //<TD> HEIGHT

if($widthscale < 1)
$nwidth = $ix*$widthscale;
else
$nwidth = $ix/$widthscale;

if($heightscale < 1)
$nheight = $iy*$heightscale;
else
$nheight = $iy/$heightscale;

?>

Correction: to find $y2 it should be...

// set y side to a proportional size
$y2 = $m * $x_max; // not $x1

Thanks Norbert =)