code: public function loadIntoExisting($pFilename, Spreadsheet $spreadsheet)
      {
          $lineEnding = ini_get('auto_detect_line_endings');
          ini_set('auto_detect_line_endings', true);
  
          // Open file
          if (!$this->canRead($pFilename)) {
              throw new Exception($pFilename . ' is an Invalid Spreadsheet file.');
          }
          $this->openFile($pFilename);
          $fileHandle = $this->fileHandle;
  
          // Skip BOM, if any
          $this->skipBOM();
          $this->checkSeparator();
          $this->inferSeparator();
  
          // Create new PhpSpreadsheet object
          while ($spreadsheet->getSheetCount() <= $this->sheetIndex) {
              $spreadsheet->createSheet();
          }
          $sheet = $spreadsheet->setActiveSheetIndex($this->sheetIndex);
  
          // Set our starting row based on whether we're in contiguous mode or not
          $currentRow = 1;
          if ($this->contiguous) {
              $currentRow = ($this->contiguousRow == -1) ? $sheet->getHighestRow() : $this->contiguousRow;
          }
  
          // Loop through each line of the file in turn
          while (($rowData = fgetcsv($fileHandle, 0, $this->delimiter, $this->enclosure)) !== false) {
              $columnLetter = 'A';
              foreach ($rowData as $rowDatum) {
                  if ($rowDatum != '' && $this->readFilter->readCell($columnLetter, $currentRow)) {
                      // Convert encoding if necessary
                      if ($this->inputEncoding !== 'UTF-8') {
                          $rowDatum = StringHelper::convertEncoding($rowDatum, 'UTF-8', $this->inputEncoding);
                      }
  
                      // Set cell value
                      $sheet->getCell($columnLetter . $currentRow)->setValue($rowDatum);
                  }
                  ++$columnLetter;
              }
              ++$currentRow;
          }
  
          // Close file
          fclose($fileHandle);
  
          if ($this->contiguous) {
              $this->contiguousRow = $currentRow;
          }
  
          ini_set('auto_detect_line_endings', $lineEnding);
  
          // Return
          return $spreadsheet;
      }
docs: Loads PhpSpreadsheet from file into PhpSpreadsheet instance.
  
  @param string $pFilename
  @param Spreadsheet $spreadsheet
  
  @throws Exception
  
  @return Spreadsheet