public function getRandomKey($length = self::TOKEN_LENGTH)
    {
        $chars = "A1B2C3D4E5F6G7H8I9J0K1L2M3N4O5P6Q7R8S9T0U1V2W3X4Y5Z6a1b2c3d4e5f6g7h8i9j0k1l2m3n4o5p6q7r8s9t0u1v2w3x4y5z6";
        $key = "";

        for ($i = 0; $i < $length; $i++) {
            $key .= $chars{mt_rand(0, strlen($chars) - 1)};
        }

        return $key;
    }