So these values can be obtained in the script like this :
$x = $_GET['x'];
$y = $_GET['y'];
$h = $_GET['h'];
$w = $_GET['w'];

For e.g. if its a jpg image then

$src = imagecreatefromjpeg($path);
where $path has the path to the image.

Now create a new image with height and width
$dest = imagecreatetruecolor($w, $h);
imagecopy($dest, $src, 0, 0, $x, $y, $w, $h);

Now overwrite the existing file :
imagejpeg($dest , $path);

You may need to do a chmod over the new file.
chmod($path , 0755);

Free memory
imagedestroy($dest);
imagedestroy($src);

Now $path is the new image which is cropped as per the values of x , y , h , w