code: public function loadIntoExisting($pFilename, Spreadsheet $spreadsheet)
      {
          // Validate
          if (!$this->canRead($pFilename)) {
              throw new Exception($pFilename . ' is an Invalid HTML file.');
          }
  
          // Create new sheet
          while ($spreadsheet->getSheetCount() <= $this->sheetIndex) {
              $spreadsheet->createSheet();
          }
          $spreadsheet->setActiveSheetIndex($this->sheetIndex);
  
          //    Create a new DOM object
          $dom = new DOMDocument();
          //    Reload the HTML file into the DOM object
          $loaded = $dom->loadHTML(mb_convert_encoding($this->securityScanFile($pFilename), 'HTML-ENTITIES', 'UTF-8'));
          if ($loaded === false) {
              throw new Exception('Failed to load ' . $pFilename . ' as a DOM Document');
          }
  
          //    Discard white space
          $dom->preserveWhiteSpace = false;
  
          $row = 0;
          $column = 'A';
          $content = '';
          $this->processDomElement($dom, $spreadsheet->getActiveSheet(), $row, $column, $content);
  
          // Return
          return $spreadsheet;
      }
docs: Loads PhpSpreadsheet from file into PhpSpreadsheet instance.
  
  @param string $pFilename
  @param Spreadsheet $spreadsheet
  
  @throws Exception
  
  @return Spreadsheet