- var_dump: documentation ( source)
- preg_match: documentation ( source)
<?php
$_content = '<iframe src="//player.vimeo.com/video/36899035" width="500" height="281" frameborder="0" webkitallowfullscreen mozallowfullscreen allowfullscreen></iframe> <p><a href="http://vimeo.com/36899035">Moving a Tree</a> from <a href="http://vimeo.com/user4457549">Avery Lawrence</a> on <a href="https://vimeo.com">Vimeo</a>.</p>';
preg_match('#vimeo.com[^\'"0-9]*([0-9]{6,10})#i',$_content, $matches);
var_dump($matches);
echo $_content;