Elgg\Menu\Service::combineMenus PHP Method

    function combineMenus(array $names = [], array $params = [], $new_name = '')
    {
        if (!$new_name) {
            $new_name = implode('__', $names);
        }
        $all_items = [];
        foreach ($names as $name) {
            $items = $this->getMenu($name, $params)->getItems();
            foreach ($items as $item) {
                $section = $item->getSection();
                if ($section == 'default') {
                    $item->setSection($name);
                }
                $item->setData('menu_name', $name);
                $all_items[] = $item;
            }
        }
        $params['name'] = $new_name;
        return new UnpreparedMenu($params, $all_items);
    }