迭代菜单的多维数组

Hello I have a menu made in codeigniter. but I also want this to have submenu's

Therefore I get an array and go through it with a foreach loop.

        <ul>
            <?php foreach ($menu_item as $menu =>& $key): ?>    
            <li><?php echo anchor($menu, $key, $this->uri->slash_segment(1, 'leading') == $menu ? 'class="active"' : '') ?></li>
            <?php endforeach ?>
        </ul>

Now the problem is that this works great if its just one menu without submenu's but when I get an array like this

$menu_item = array(
    '/' => 'Home',
    '/about' => 'About',
    '/foo' => 'boo',
    '/contact' => 'contact',
    'test' => array(
               'foo' => 'boo'
              ),

    'test2' => 'foo2'
);

Than it doesn't work anymore. How can I loop through everything and output it as a good menu?