code: private function writeMergeCells(XMLWriter $objWriter, PhpspreadsheetWorksheet $pSheet)
      {
          if (count($pSheet->getMergeCells()) > 0) {
              // mergeCells
              $objWriter->startElement('mergeCells');
  
              // Loop mergeCells
              foreach ($pSheet->getMergeCells() as $mergeCell) {
                  // mergeCell
                  $objWriter->startElement('mergeCell');
                  $objWriter->writeAttribute('ref', $mergeCell);
                  $objWriter->endElement();
              }
  
              $objWriter->endElement();
          }
      }
docs: Write MergeCells.
  
  @param XMLWriter $objWriter XML Writer
  @param PhpspreadsheetWorksheet $pSheet Worksheet