code: private function readMergedCells()
      {
          $length = self::getUInt2d($this->data, $this->pos + 2);
          $recordData = $this->readRecordData($this->data, $this->pos + 4, $length);
  
          // move stream pointer to next record
          $this->pos += 4 + $length;
  
          if ($this->version == self::XLS_BIFF8 && !$this->readDataOnly) {
              $cellRangeAddressList = $this->readBIFF8CellRangeAddressList($recordData);
              foreach ($cellRangeAddressList['cellRangeAddresses'] as $cellRangeAddress) {
                  if ((strpos($cellRangeAddress, ':') !== false) &&
                      ($this->includeCellRangeFiltered($cellRangeAddress))) {
                      $this->phpSheet->mergeCells($cellRangeAddress);
                  }
              }
          }
      }
docs: MERGEDCELLS.
  
  This record contains the addresses of merged cell ranges
  in the current sheet.
  
  --    "OpenOffice.org's Documentation of the Microsoft
  Excel File Format"