framework/grid/GridView.php
public function renderColumnGroup()
    {
        foreach ($this->columns as $column) {
            /* @var $column Column */
            if (!empty($column->options)) {
                $cols = [];
                foreach ($this->columns as $col) {
                    $cols[] = Html::tag('col', '', $col->options);
                }

                return Html::tag('colgroup', implode("\n", $cols));
            }
        }

        return false;
    }
public function renderColumnGroup()
    {
        foreach ($this->columns as $column) {
            /* @var $column Column */
            if (!empty($column->options)) {
                $cols = [];
                foreach ($this->columns as $col) {
                    $cols[] = Html::tag('col', '', $col->options);
                }

                return Html::tag('colgroup', implode("\n", $cols));
            }
        }

        return false;
    }
Renders the column group HTML.
@return bool|string the column group HTML or `false` if no column group should be rendered.