ul-li-ul nested menu

// creating ul-li-ul nested menu using only depth level of next element
$lists = array(1,1,2,2,2,1,1,2,3,3,2,1);
$res = "";
$prev = 0;
foreach ($lists as $list) {
    if ($list>$prev) $res .= "<ul>\n";
    else $res .= "</li>\n";
    if ($list<$prev) $res .= "</ul></li>\n";
    $res .= '<li>'.$list;
    $prev = $list;
}
$res .= "</li></ul>\n";
echo $res;