protected function _encodeInt24($integer)
    {
        $integer = (int) $integer;
        if ($integer < 0 || $integer > 0xffffff) {
            throw new InvalidArgumentException('Integer is out of range');
        }

        $string  = $this->_itoa64[$integer & 0x3f];
        $string .= $this->_itoa64[($integer >> 0x06) & 0x3f];
        $string .= $this->_itoa64[($integer >> 0x0c) & 0x3f];
        $string .= $this->_itoa64[($integer >> 0x12) & 0x3f];

        return $string;
    }