src/collections/Menu.php
protected function renderMenu($items, $parentItem)
    {
        $options = ArrayHelper::getValue($parentItem, 'options');
        $label = $this->getLabel($parentItem);
        $items = Html::tag('div', $this->renderItems($items), ['class' => 'menu']);
        Html::addCssClass($options, 'ui');
        Html::addCssClass($options, 'header');
        return
            Html::tag(
                'div',
                Html::tag('div', $label, $options) . $items,
                ['class' => 'item']
            );
    }
protected function renderMenu($items, $parentItem)
    {
        $options = ArrayHelper::getValue($parentItem, 'options');
        $label = $this->getLabel($parentItem);
        $items = Html::tag('div', $this->renderItems($items), ['class' => 'menu']);
        Html::addCssClass($options, 'ui');
        Html::addCssClass($options, 'header');
        return
            Html::tag(
                'div',
                Html::tag('div', $label, $options) . $items,
                ['class' => 'item']
            );
    }
Renders the given items as a dropdown.
This method is called to create sub-menus.

@param array $items the given items. Please refer to [[Dropdown::items]] for the array structure.
@param array $parentItem the parent item information. Please refer to [[items]] for the structure of this array.

@return string the rendering result.