public static function substr_replace($str, $repl, $start, $length = null)
    {
        preg_match_all('/./us', $str, $ar);
        preg_match_all('/./us', $repl, $rar);
        if ($length === null) {
            $length = AnHelperString::strlen($str);
        }
        array_splice($ar[0], $start, $length, $rar[0]);
        return join('', $ar[0]);
    }