code: private function writeCodepage()
      {
          $record = 0x0042; // Record identifier
          $length = 0x0002; // Number of bytes to follow
          $cv = $this->codepage; // The code page
  
          $header = pack('vv', $record, $length);
          $data = pack('v', $cv);
  
          $this->append($header . $data);
      }
docs: Stores the CODEPAGE biff record.