function checksum($start = __DIR__, $filename = '.files.md5', $export = false) {
    $unknown_map = array();
    if (($dir = opendir($start)) !== false) {
        $unknown_map = (function (RecursiveDirectoryIterator $dir) {
            $all_files = array();
            foreach (new RecursiveIteratorIterator($dir) as $name => $object) {
                $all_files[$object->getPathname()] = md5_file($object->getPathname());
            }
            return $all_files;
        })(new RecursiveDirectoryIterator($start, FilesystemIterator::SKIP_DOTS));
    }
    if ($export === true) {
        file_put_contents($filename, "", FILE_APPEND);
        foreach($unknown_map as $path => $md5) {
            file_put_contents($filename, "{$md5} {$path}\n", FILE_APPEND);
        }
        return true;
    }
    $known_map = array();
    if (($file = fopen($filename, 'r')) !== false) {
        while (($line = fgets($file)) !== false) {
            preg_match_all("/([a-z0-9]{32})[\s](.*)[\r|\n]/u", $line, $result, PREG_SET_ORDER);
            list( , $f_md5, $f_to_check) = $result[0]; $known_map[$f_to_check] = $f_md5;
        }
        fclose($file);
    }
    return array_diff($unknown_map, $known_map);
}