code: public function getHighestColumn($row = null)
      {
          if ($row == null) {
              $colRow = $this->getHighestRowAndColumn();
  
              return $colRow['column'];
          }
  
          $columnList = [1];
          foreach ($this->getCoordinates() as $coord) {
              sscanf($coord, '%[A-Z]%d', $c, $r);
              if ($r != $row) {
                  continue;
              }
              $columnList[] = Coordinate::columnIndexFromString($c);
          }
  
          return Coordinate::stringFromColumnIndex(max($columnList) + 1);
      }
docs: Get highest worksheet column.
  
  @param string $row Return the highest column for the specified row,
  or the highest column of any row if no row number is passed
  
  @return string Highest column name