code: private function convertRef2d($cell)
      {
          // Convert the cell reference
          $cell_array = $this->cellToPackedRowcol($cell);
          list($row, $col) = $cell_array;
  
          // The ptg value depends on the class of the ptg.
          $ptgRef = pack('C', $this->ptg['ptgRefA']);
  
          return $ptgRef . $row . $col;
      }
docs: Convert an Excel reference such as A1, $B2, C$3 or $D$4 to a ptgRefV.
  
  @param string $cell An Excel cell reference
  
  @return string The cell in packed() format with the corresponding ptg