/** уворовал отсюда http://snipplr.com/view/12981/php-create-drop-shadow-on-image/ **/

/* offset of drop shadow from top left */
define("DS_OFFSET",  5);
 
/* number of steps from black to background color */
define("DS_STEPS", 10);

/* distance between steps */
define("DS_SPREAD", 1);

/* define the background color */
$background = array("r" => 255, "g" => 255, "b" => 255);

$src = '/tmp/a.jpg';

if(isset($src) && file_exists($src)) {
     /* create a new canvas.  New canvas dimensions should be larger than the original's */
     list($o_width, $o_height) = getimagesize($src);
     $width  = $o_width + DS_OFFSET;
     $height = $o_height + DS_OFFSET;
     $image = imagecreatetruecolor($width, $height);

     /* determine the offset between colors */
     $step_offset = array("r" => ($background["r"] / DS_STEPS), "g" => ($background["g"] / DS_STEPS), "b" => ($background["b"] / DS_STEPS));

     /* calculate and allocate the needed colors */
     $current_color = $background;
     for ($i = 0; $i <= DS_STEPS; $i++) {
     $colors[$i] = imagecolorallocate($image, round($current_color["r"]), round($current_color["g"]), round($current_color["b"]));

     $current_color["r"] -= $step_offset["r"];
     $current_color["g"] -= $step_offset["g"];
     $current_color["b"] -= $step_offset["b"];
     }

     /* floodfill the canvas with the background color */
     imagefilledrectangle($image, 0,0, $width, $height, $colors[0]);

     /* draw overlapping rectangles to create a drop shadow effect */
     for ($i = 0; $i < count($colors); $i++) {
     imagefilledrectangle($image, DS_OFFSET, DS_OFFSET, $width, $height, $colors[$i]);
     $width -= DS_SPREAD;
     $height -= DS_SPREAD;
     }

     /* overlay the original image on top of the drop shadow */
     $original_image = imagecreatefromjpeg($src);
     imagecopymerge($image, $original_image, 0,0, 0,0, $o_width, $o_height, 100);

     /* output the image */
     header("Content-type: image/jpeg");
     imagejpeg($image, "", 100);

     /* clean up the image resources */
     imagedestroy($image);
     imagedestroy($original_image);
}