code: protected function buildTableFooter($keys)
      {
          $haveTotal = false;
          $cells = array();
          foreach ($keys as $key) {
              $colHasTotal = isset($this->tableInfo['totals'][$key]);
              $cells[] = $colHasTotal
                  ? $this->dump(\round($this->tableInfo['totals'][$key], 6), true, 'td')
                  : '<td></td>';
              $haveTotal = $haveTotal || $colHasTotal;
          }
          if (!$haveTotal) {
              return '';
          }
          return '<tfoot>'."\n"
              .'<tr><td>&nbsp;</td>'
                  .($this->tableInfo['haveObjRow'] ? '<td>&nbsp;</td>' : '')
                  .\implode('', $cells)
              .'</tr>'."\n"
              .'</tfoot>'."\n";
      }
docs: Returns table's tfoot
  
  @param array $keys column header values (keys of array or property names)
  
  @return string