code: public function writeRelationships(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');
  
          $customPropertyList = $spreadsheet->getProperties()->getCustomProperties();
          if (!empty($customPropertyList)) {
              // Relationship docProps/app.xml
              $this->writeRelationship(
                  $objWriter,
                  4,
                  'http://schemas.openxmlformats.org/officeDocument/2006/relationships/custom-properties',
                  'docProps/custom.xml'
              );
          }
  
          // Relationship docProps/app.xml
          $this->writeRelationship(
              $objWriter,
              3,
              'http://schemas.openxmlformats.org/officeDocument/2006/relationships/extended-properties',
              'docProps/app.xml'
          );
  
          // Relationship docProps/core.xml
          $this->writeRelationship(
              $objWriter,
              2,
              'http://schemas.openxmlformats.org/package/2006/relationships/metadata/core-properties',
              'docProps/core.xml'
          );
  
          // Relationship xl/workbook.xml
          $this->writeRelationship(
              $objWriter,
              1,
              'http://schemas.openxmlformats.org/officeDocument/2006/relationships/officeDocument',
              'xl/workbook.xml'
          );
          // a custom UI in workbook ?
          if ($spreadsheet->hasRibbon()) {
              $this->writeRelationShip(
                  $objWriter,
                  5,
                  'http://schemas.microsoft.com/office/2006/relationships/ui/extensibility',
                  $spreadsheet->getRibbonXMLData('target')
              );
          }
  
          $objWriter->endElement();
  
          return $objWriter->getData();
      }
docs: Write relationships to XML format.
  
  @param Spreadsheet $spreadsheet
  
  @throws WriterException
  
  @return string XML Output