foreach里面的unset

function subtree($arr,$parent){

    $subs = array();

    $task = array($parent);

    while(!empty($task)){

        $flag = false;                               //$flag初始化为假

        foreach($arr as $k=>$v){

            if($v['parent'] == $parent){

                $subs[] = $v;

                $parent = $v['id'];

                array_push($task,$v['id']);

                unset($arr[$k]);                      //为什么此处要用unset??????

                $flag = true;                        //找到子栏目

            }

        }

        if($flag == false){

            array_pop($task);

            $parent = end($task);

        }

        print_r($task);

    }

    return $subs;

}