protected static function bits2hex($bits) 
    {
        $hex   = '';
        $total = strlen($bits) / 4;

        for ($i = 0; $i < $total; $i++) {
            $nibbel = substr($bits, $i*4, 4);

            $pos  = strpos(self::$_bin, $nibbel);
            $hex .= substr(self::$_hex, $pos/5, 1);
        }

        return $hex;
    }