Mine

function rTree($_cats=null) { 
    static $i,$cats,$count,$path; 
    $string = ""; 
    if(is_array($_cats)) { 
        $cats = $_cats; 
        $count = count($cats); 
        $i = -1; 
        $path = array(); 
    } 
    while($i++<$count-1) { 
        $path[] = $cats[$i]['short_name']; 
        $string .= '<li><a href="/'.join('/',$path).'/'.$cats[$i]['category_id'].'/">'. 
            $cats[$i]['category'].'</a>'; 
        if($count-$i==1 or $cats[$i]['rgt']+1<$cats[$i+1]['lft']) { 
            break; 
        } else if($cats[$i]['rgt']>$cats[$i+1]['lft']) { 
            $string .= '<ul>'.rTree().'</ul>'; 
        } 
        $string .= "</li>"; 
        array_pop($path); 
    } 
    return $string; 
}