code: public function writeHeaderFooterDrawingRelationships(\PhpOffice\PhpSpreadsheet\Worksheet\Worksheet $pWorksheet)
      {
          // 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');
  
          // Loop through images and write relationships
          foreach ($pWorksheet->getHeaderFooter()->getImages() as $key => $value) {
              // Write relationship for image drawing
              $this->writeRelationship(
                  $objWriter,
                  $key,
                  'http://schemas.openxmlformats.org/officeDocument/2006/relationships/image',
                  '../media/' . $value->getIndexedFilename()
              );
          }
  
          $objWriter->endElement();
  
          return $objWriter->getData();
      }
docs: Write header/footer drawing relationships to XML format.
  
  @param \PhpOffice\PhpSpreadsheet\Worksheet\Worksheet $pWorksheet
  
  @throws WriterException
  
  @return string XML Output