code: private function writeRecalcId()
      {
          $record = 0x01C1; // Record identifier
          $length = 8; // Number of bytes to follow
  
          $header = pack('vv', $record, $length);
  
          // by inspection of real Excel files, MS Office Excel 2007 writes this
          $data = pack('VV', 0x000001C1, 0x00001E667);
  
          return $this->writeData($header . $data);
      }
docs: Write the RECALCID record.
  
  @return string