code: private function convertRef3d($cell)
      {
          // Split the ref at the ! symbol
          list($ext_ref, $cell) = explode('!', $cell);
  
          // Convert the external reference part (different for BIFF8)
          $ext_ref = $this->getRefIndex($ext_ref);
  
          // Convert the cell reference part
          list($row, $col) = $this->cellToPackedRowcol($cell);
  
          // The ptg value depends on the class of the ptg.
          $ptgRef = pack('C', $this->ptg['ptgRef3dA']);
  
          return $ptgRef . $ext_ref . $row . $col;
      }
docs: Convert an Excel 3d reference such as "Sheet1!A1" or "Sheet1:Sheet2!A1" to a
  ptgRef3d.
  
  @param string $cell An Excel cell reference
  
  @return mixed the packed ptgRef3d token on success