static public function isSubscriptionType($subscriptionType) {
    return ((string) $subscriptionType == Connect2PayClient::SUBSCRIPTION_TYPE_NORMAL ||
        (string) $subscriptionType == Connect2PayClient::SUBSCRIPTION_TYPE_INFINITE ||
        (string) $subscriptionType == Connect2PayClient::SUBSCRIPTION_TYPE_ONETIME ||
        (string) $subscriptionType == Connect2PayClient::SUBSCRIPTION_TYPE_LIFETIME);
  }