php 递归 返回数组

private function tree_data(&$list, $parent){
            $tree = array();
            foreach($list as $row) {
                if($row['permission_parent_id'] == $parent) {

                    $row['children'] = $this->tree_data($list, $row['permission_id']);
                    $tree[] = $row;
                }
            }
            return $tree;
}