最基本的无限极分类

function tree(&$list,$pid=0,$level=0,$html='--'){
    static $tree = array();
    foreach($list as $v){
        if($v['pid'] == $pid){
            $v['sort'] = $level;
            $v['html'] = str_repeat($html,$level);
            $tree[] = $v;
            tree($list,$v['id'],$level+1);
        } 
    }
    return $tree;
}