code: private function writeDimensions()
      {
          $record = 0x0200; // Record identifier
  
          $length = 0x000E;
          $data = pack('VVvvv', $this->firstRowIndex, $this->lastRowIndex + 1, $this->firstColumnIndex, $this->lastColumnIndex + 1, 0x0000); // reserved
  
          $header = pack('vv', $record, $length);
          $this->append($header . $data);
      }
docs: Writes Excel DIMENSIONS to define the area in which there is data.