protected static function hex2bits($hex) 
    {
        $bits  = '';
        $total = strlen($hex);

        for ($i = 0; $i < $total; $i++) {
            $pos   = strpos(self::$_hex, $hex[$i]);
            $bits .= substr(self::$_bin, $pos*5, 4);
        }

        return $bits;
    }