PHP无限级分类方法

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function get_tree( &$tree = array() ){
        if (empty($tree)) {
            $tree = M("category")->where("pid = 0")->select();
        }
        //循环父级分类,查找子分类
        foreach ($tree as $key => $value) {
            $temp = M("category")->where("pid = ".$value['id'])->select();//查找子分类
            if ($temp) {
                $tree[$key]['_child'] = $temp;
                $this->get_tree($tree[$key]['_child']);
            }
        }
        return $tree;
    }
}