- preg_replace: documentation ( source)
<?php
$_content = '<div><div><iframe width="1280" height="720" src="//www.youtube.com/embed/NKvxWMsGW9E?rel=0" frameborder="0" allowfullscreen>blablabla<img src="someimage.gif"></iframe></div></div><div>this div is ok</div>';
$_content = preg_replace('#(?:<(?:div|span)[^>]*>[\s]*)*(<iframe[\s]*(?:[^>]+)>(?:.*?)<\/iframe[^>]*>)(?:[\s]*<(?:embed|script)[^>]*>[^<]*<\/(?:embed|script)[^>]*>[\s]*)*(?:<\/(?:div|span)[^>]*>)*#iu', '$1', $_content);
echo $_content;
//var_dump($matches);