quindi

$string = "http://external.ak.fbcdn.net/safe_image.php?d=eb3dc128bfe599cceecadebe5dde16e9& w=130&h=130&url=http%3A%2F%2Fi56.tinypic.com%2Fnlv 3m0.jpg";

come faccio a determinare lo:
$length e start??

substr ($string , $start, $length)