code: private function writeProtect()
      {
          // Exit unless sheet protection has been specified
          if (!$this->phpSheet->getProtection()->getSheet()) {
              return;
          }
  
          $record = 0x0012; // Record identifier
          $length = 0x0002; // Bytes to follow
  
          $fLock = 1; // Worksheet is protected
  
          $header = pack('vv', $record, $length);
          $data = pack('v', $fLock);
  
          $this->append($header . $data);
      }
docs: Set the Biff PROTECT record to indicate that the worksheet is protected.