<?php
function shutdown() {
global $orig_image, $cropped_image, $resized_image;
if($orig_image) { imagedestroy($orig_image); }
if($cropped_image) { imagedestroy($cropped_image); }
if($resized_image) { imagedestroy($resized_image); }
}
register_shutdown_function("shutdown");
list($old_width, $old_height) = getimagesize("./". $_GET['image']);
if(($_GET['width'] / $_GET['height']) > ($old_width / $old_height)) {
$new_width = $_GET['width'];
$new_height = $old_height * ($_GET['width'] / $old_width);
} else {
$new_width = $old_width * ($_GET['height'] / $old_height);
$new_height = $_GET['height'];
}
$resized_image = imagecreatetruecolor($new_width, $new_height);
$orig_image = imagecreatefromgif("./". $_GET['image']);
imagecopyresampled($resized_image, $orig_image, 0, 0, 0, 0, $new_width, $new_height, $old_width, $old_height);
$cropped_image = imagecreatetruecolor($_GET['width'], $_GET['height']);
imagecopy($cropped_image, $resized_image, 0, 0, 0, 0, imagesx($resized_image), imagesy($resized_image));
$black = imagecolorallocate($cropped_image, 0, 0, 0);
imagerectangle($cropped_image, 0, 0, $_GET['width'] - 1, $_GET['height'] - 1, $black);
header("Content-type: image/png");
imagepng($cropped_image);
?>
 
To create a thumbnail with it:
 
<img src="path/to/thumbnailer.php?image=image.gif&width=50&height=50" alt="" />