- var_dump: documentation ( source)
- preg_match: documentation ( source)
- current: documentation ( source)
<?php
$images[] = "/cache/user-ErgL4WpHE4SGK7pc-2014-09-17_10-10-18-questionmark.png";
preg_match('/-([a-zA-Z0-9]*).png/', (string)current($images), $matches);
var_dump($matches);