function list_to_tree($list, $pk='id',$pid = 'pid',$child = '_child',$root=0) {    // 创建Tree    $tree = array();    if(is_array($list)) {        // 创建基于主键的数组引用        $refer = array();        foreach ($list as $key => $data) {            $refer[$data[$pk]] =& $list[$key];        }        foreach ($list as $key => $data) {            // 判断是否存在parent            $parentId = $data[$pid];            if ($root == $parentId) {                $tree[] =& $list[$key];            }else{                if (isset($refer[$parentId])) {                    $parent =& $refer[$parentId];                    $parent[$child][] =& $list[$key];                }            }        }    }    return $tree;}