- 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>';
preg_match('#(?<=(?:v|i)=)[a-zA-Z0-9-]+(?=&)|(?<=(?:v|i)\/)[^&\n]+|(?<=embed\/)[^"&\n]+|(?<=(?:v|i)=)[^&\n]+|(?<=youtu.be\/)[^&\n]+#', $string, $matches);
var_dump($matches);