public function treeAsHtml() {
    $tree = $this->getTree();
    $html = "<ul>";
    for ($i=0; $i<count($tree); $i++) {
        $html .= "<li>" . $tree[$i]['name'];
        if ($tree[$i]['level'] < $tree[$i+1]['level']) {
            $html .= "<ul>";
        } elseif ($tree[$i]['level'] == $tree[$i+1]['level']) {
            $html .= "</li>";
        } else {
            $diff = $tree[$i]['level'] - $tree[$i+1]['level'];
            $html .= str_repeat("</li></ul>", $diff) . "</li>";
        }
    }
    $html .= "</ul>";
    return $html;
}