code: public function write(Spreadsheet $spreadsheet = null)
      {
          $objWriter = null;
          if ($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');
  
          // Settings
          $objWriter->startElement('office:document-settings');
          $objWriter->writeAttribute('xmlns:office', 'urn:oasis:names:tc:opendocument:xmlns:office:1.0');
          $objWriter->writeAttribute('xmlns:xlink', 'http://www.w3.org/1999/xlink');
          $objWriter->writeAttribute('xmlns:config', 'urn:oasis:names:tc:opendocument:xmlns:config:1.0');
          $objWriter->writeAttribute('xmlns:ooo', 'http://openoffice.org/2004/office');
          $objWriter->writeAttribute('office:version', '1.2');
  
          $objWriter->startElement('office:settings');
          $objWriter->startElement('config:config-item-set');
          $objWriter->writeAttribute('config:name', 'ooo:view-settings');
          $objWriter->startElement('config:config-item-map-indexed');
          $objWriter->writeAttribute('config:name', 'Views');
          $objWriter->endElement();
          $objWriter->endElement();
          $objWriter->startElement('config:config-item-set');
          $objWriter->writeAttribute('config:name', 'ooo:configuration-settings');
          $objWriter->endElement();
          $objWriter->endElement();
          $objWriter->endElement();
  
          return $objWriter->getData();
      }
docs: Write settings.xml to XML format.
  
  @param Spreadsheet $spreadsheet
  
  @throws \PhpOffice\PhpSpreadsheet\Writer\Exception
  
  @return string XML Output