code: private function createVoidRequest($shipmentData)
      {
          $xml = new DOMDocument();
          $xml->formatOutput = true;
  
          $container = $xml->appendChild($xml->createElement('VoidShipmentRequest'));
          $request = $container->appendChild($xml->createElement('Request'));
  
          $node = $xml->importNode($this->createTransactionNode(), true);
          $request->appendChild($node);
  
          $request->appendChild($xml->createElement('RequestAction', '1'));
  
          if (is_string($shipmentData)) {
              $container->appendChild($xml->createElement('ShipmentIdentificationNumber', strtoupper($shipmentData)));
          } else {
              $expanded = $container->appendChild($xml->createElement('ExpandedVoidShipment'));
              $expanded->appendChild($xml->createElement('ShipmentIdentificationNumber', strtoupper($shipmentData['shipmentId'])));
  
              if (array_key_exists('trackingNumbers', $shipmentData)) {
                  foreach ($shipmentData['trackingNumbers'] as $tn) {
                      $expanded->appendChild($xml->createElement('TrackingNumber', strtoupper($tn)));
                  }
              }
          }
  
          return $xml->saveXML();
      }
docs: Creates a void shipment request.
  
  @param string|array $shipmentData
  
  @return string