<?php

$re = '~^Location:.*/([^_]*)~'; 
$str = "Location:http://s...content-available-to-author-only...e.com/something/folder1/folder2/folder3/folder4/folder5/20385425_632383970568166e82391f.png"; 
preg_match($re, $str, $matches);
print_r($matches[1]);