code: protected function storeBof($type)
      {
          $record = 0x0809; // Record identifier    (BIFF5-BIFF8)
          $length = 0x0010;
  
          // by inspection of real files, MS Office Excel 2007 writes the following
          $unknown = pack('VV', 0x000100D1, 0x00000406);
  
          $build = 0x0DBB; //    Excel 97
          $year = 0x07CC; //    Excel 97
  
          $version = 0x0600; //    BIFF8
  
          $header = pack('vv', $record, $length);
          $data = pack('vvvv', $version, $type, $build, $year);
          $this->append($header . $data . $unknown);
      }
docs: Writes Excel BOF record to indicate the beginning of a stream or
  sub-stream in the BIFF file.
  
  @param int $type type of BIFF file to write: 0x0005 Workbook,
  0x0010 Worksheet