function create_menu(&$tree, $id) {
    $html = '';
    if (isset($tree[$id]['children'])) {
        foreach ($tree[$id]['children'] as $t) {
            $html .= '<li>';
            $html .= "<a href='content/show/{$t['id']}' title='{$t['title']}'>";
            $html .= $t['title'];
            $html .= "</a>";
            $html .= '</li>';
        }
    } else {
        $html .= '<li></li>';
    }
    return $html;
}