Someone

function getoldestfile($directory, $entry) {
    if ($handle = opendir($directory)) {
        while (false !== ($file = readdir($handle))) {
            if (is_file($directory.$file)) {
                $files[] = $file;
            }
                    }
        if (count($files) <= 12) {
            return;
        } else {
            foreach ($files as $val) {
                if (is_file($directory.$val)) {
                	preg_match("/".$entry."/i",  $val, $finds);
                    $file_date[$finds] = filemtime($directory.$finds);
                }
            }
        }
    }
    closedir($handle);
    asort($file_date, SORT_NUMERIC);
    reset($file_date);
    $oldest = key($file_date);
    return $oldest;
}
echo getoldestfile("/var/www/htdocs/files/du/tmp/",40);