require_once('yubicloud.class.php');
$yubicloud = new Yubicloud("my_client_id", "my_secret_key");
$result = $yubicloud->CheckOnYubiCloud($otp_to_check);

                     OK  The OTP is valid.
                BAD_OTP  The OTP is invalid format.
           REPLAYED_OTP  The OTP has already been seen by the service.
          BAD_SIGNATURE  The HMAC signature verification failed.
      MISSING_PARAMETER  The request lacks a parameter.
         NO_SUCH_CLIENT  The request id does not exist.
  OPERATION_NOT_ALLOWED  The request id is not allowed to verify OTPs.
          BACKEND_ERROR  Unexpected error in Yubico servers. Please contact them if you see this error.
     NOT_ENOUGH_ANSWERS  Server could not get requested number of syncs during before timeout.
       REPLAYED_REQUEST  Server has seen the OTP/Nonce combination before.
              BAD_NONCE  Answer Nonce is different from the request Nonce.
       CONNECTION_ERROR  Impossible to make a connection with the YubiCloud servers.
       OTP_IS_DIFFERENT  Answer OTP is different from request OTP.
     OUT_OF_TIME_WINDOW  Timestamp difference with the Yubico servers is bigger than yubicloud_max_time_window.
         SERVER_TIMEOUT  Timeout while waiting an answer from the server.