<?php
$str='<iframe width="560" height="315" src="http://www.youtube.com/embed/7VopPJIWIDc" frameborder="0" allowfullscreen></iframe>';
$DOM = new DOMDocument;
$DOM->loadHTML($str); // Your string
//get all anchors
$anchors = $DOM->getElementsByTagName('iframe');
//display all hrefs
for ($i = 0; $i < $anchors->length; $i++){
echo $anchors->item($i)->getAttribute('src');
}
?>
You can check if the node has a srcusing hasAttribute() first if necessary.