code: private function createAcceptRequest($shipmentDigest)
      {
          $xml = new DOMDocument();
          $xml->formatOutput = true;
  
          $container = $xml->appendChild($xml->createElement('ShipmentAcceptRequest'));
          $request = $container->appendChild($xml->createElement('Request'));
  
          $node = $xml->importNode($this->createTransactionNode(), true);
          $request->appendChild($node);
  
          $request->appendChild($xml->createElement('RequestAction', 'ShipAccept'));
          $container->appendChild($xml->createElement('ShipmentDigest', $shipmentDigest));
  
          return $xml->saveXML();
      }
docs: Creates a ShipAccept request.
  
  @param string $shipmentDigest
  
  @return string