code: private function cellToPackedRowcol($cell)
      {
          $cell = strtoupper($cell);
          list($row, $col, $row_rel, $col_rel) = $this->cellToRowcol($cell);
          if ($col >= 256) {
              throw new WriterException("Column in: $cell greater than 255");
          }
          if ($row >= 65536) {
              throw new WriterException("Row in: $cell greater than 65536 ");
          }
  
          // Set the high bits to indicate if row or col are relative.
          $col |= $col_rel << 14;
          $col |= $row_rel << 15;
          $col = pack('v', $col);
  
          $row = pack('v', $row);
  
          return [$row, $col];
      }
docs: pack() row and column into the required 3 or 4 byte format.
  
  @param string $cell The Excel cell reference to be packed
  
  @return array Array containing the row and column in packed() format