function encode($appNumber)
    {
        $secretKey = md5("cc1fa18c99877f15f8c9f15169a710c8");
        $value = "cid=85&external_reference=".$appNumber."&optional_identifier=";

        return rtrim(
            bin2hex(
                mcrypt_encrypt(
                    MCRYPT_RIJNDAEL_256,
                    $secretKey,
                    $value,
                    MCRYPT_MODE_ECB,
                    mcrypt_create_iv(
                        mcrypt_get_iv_size(
                            MCRYPT_RIJNDAEL_256,
                            MCRYPT_MODE_CBC
                        )
                    )
                )
            ),
            "\0"
        );

    }