- var_dump: documentation ( source)
- preg_match: documentation ( source)
<?php
$content = '<iframe class="vine-embed" src="https://vine.co/v/Mlr5Kt22TKA/embed/simple" width="600" height="600" frameborder="0"></iframe><script async src="//platform.vine.co/static/scripts/embed.js" charset="utf-8"></script>';
preg_match('#(?<=vine.co/v/)([0-9A-Za-z])+#', $content, $matches);
var_dump($matches);