code: public function writeVBARelationships(Spreadsheet $spreadsheet)
      {
          // Create XML writer
          $objWriter = null;
          if ($this->getParentWriter()->getUseDiskCaching()) {
              $objWriter = new XMLWriter(XMLWriter::STORAGE_DISK, $this->getParentWriter()->getDiskCachingDirectory());
          } else {
              $objWriter = new XMLWriter(XMLWriter::STORAGE_MEMORY);
          }
  
          // XML header
          $objWriter->startDocument('1.0', 'UTF-8', 'yes');
  
          // Relationships
          $objWriter->startElement('Relationships');
          $objWriter->writeAttribute('xmlns', 'http://schemas.openxmlformats.org/package/2006/relationships');
          $objWriter->startElement('Relationship');
          $objWriter->writeAttribute('Id', 'rId1');
          $objWriter->writeAttribute('Type', 'http://schemas.microsoft.com/office/2006/relationships/vbaProjectSignature');
          $objWriter->writeAttribute('Target', 'vbaProjectSignature.bin');
          $objWriter->endElement();
          $objWriter->endElement();
  
          return $objWriter->getData();
      }
docs: Write relationships for a signed VBA Project.
  
  @param Spreadsheet $spreadsheet
  
  @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
  
  @return string XML Output