code: protected function getEbayHeaders($operationName)
      {
          $headers = [];
  
          // Add required headers first.
          $headers[self::HDR_API_VERSION] = $this->getConfig('apiVersion');
          $headers[self::HDR_APP_ID] = $this->getConfig('credentials')->getAppId();
          $headers[self::HDR_OPERATION_NAME] = $operationName;
          $headers[self::HDR_REQUEST_FORMAT] = 'XML';
  
          // Add optional headers.
          // Take into account siteId is an integer that can be set to zero.
          if ($this->getConfig('siteId') !== null) {
              $headers[self::HDR_SITE_ID] = $this->getConfig('siteId');
          }
  
          if ($this->getConfig('affiliateUserId')) {
              $headers[self::HDR_AFFILIATE_USER_ID] = $this->getConfig('affiliateUserId');
          }
  
          if ($this->getConfig('trackingId')) {
              $headers[self::HDR_TRACKING_ID] = $this->getConfig('trackingId');
          }
  
          if ($this->getConfig('trackingPartnerCode')) {
              $headers[self::HDR_TRACKING_PARTNER_CODE] = $this->getConfig('trackingPartnerCode');
          }
  
          return $headers;
      }
docs: Builds the needed eBay HTTP headers.
  
  @param string $operationName The name of the operation been called.
  
  @return array An associative array of eBay http headers.