- var_dump: documentation ( source)
- preg_match: documentation ( source)
<?php
$string = '<iframe width="640" height="360" src="http://www.youtube.com/embed/h9jOaIB1VGo?rel=0&wmode=transparent" frameborder="0" allowfullscreen></iframe>';
var_dump(preg_match("/src=\"(.*)\"/", $string, $matches));