code: private function _convertCreateSubscription($request) {
  
          $parameters = array();
          $parameters['Action'] = 'CreateSubscription';
          if ($request->isSetSellerId()) {
              $parameters['SellerId'] =  $request->getSellerId();
          }
          if ($request->isSetMWSAuthToken()) {
              $parameters['MWSAuthToken'] =  $request->getMWSAuthToken();
          }
          if ($request->isSetMarketplaceId()) {
              $parameters['MarketplaceId'] =  $request->getMarketplaceId();
          }
          if ($request->isSetSubscription()) {
              $SubscriptionCreateSubscriptionInput = $request->getSubscription();
              foreach  ($SubscriptionCreateSubscriptionInput->getNotificationType() as $NotificationTypeSubscriptionIndex => $NotificationTypeSubscription) {
                  $parameters['Subscription' . '.' . 'NotificationType' . '.'  . ($NotificationTypeSubscriptionIndex + 1)] =  $NotificationTypeSubscription;
              }
          }
  
          return $parameters;
      }
docs: Convert CreateSubscriptionInput to name value pairs