- explode: documentation ( source)
<?php
$str = "http://external.ak.fbcdn.net/safe_image.php?d=AQD-PsVh6CG6Q2RK&w=154&h=154&url=https\u00253A\u00252F\u00252Ffbcdn-sphotos-b-a.akamaihd.net\u00252Fhphotos-ak-prn1\u00252Ft1\u00252F1623737_673137756072004_537520808_n.jpg";
$arr = explode("url=", $str);
echo $arr[1];