php两个for循环实现无限级分类

这种方法是很常见、很传统的一种，先看表结构

表:category id int 主键,自增 name varchar 分类名称 pid int 父类id，默认0
顶级分类的 pid 默认就是0了。当我们想取出某个分类的子分类树的时候，基本思路就是递归，当然，出于效率问题不建议每次递归都查询数据库，通常的做法是先讲所有分类取出来，保存到PHP数组里，再进行处理，最后还可以将结果缓存起来以提高下次请求的效率。
先来构建一个原始数组，这个直接从数据库中拉出来就行：

$categories = array(
    array("id"=>1,"name"=>"电脑","pid"=>0),
    array("id"=>2,"name"=>"手机","pid"=>0),
    array("id"=>3,"name"=>"笔记本","pid"=>1),
    array("id"=>4,"name"=>"台式机","pid"=>1),
    array("id"=>5,"name"=>"智能机","pid"=>2),
    array("id"=>6,"name"=>"功能机","pid"=>2),
    array("id"=>7,"name"=>"超级本","pid"=>3),
    array("id"=>8,"name"=>"游戏本","pid"=>3),
);

目标是将它转化为下面这种结构
电脑 —笔记本 ——-超级本 ——-游戏本 —台式机 手机 —智能机 —功能机
用数组来表示的话，可以增加一个 children 键来存储它的子分类：

```
array(
    //1对应id，方便直接读取
    1 => array(
        "id"=>1,
        "name"=>"电脑",
        "pid"=>0,
        children=>array(
            &array(
                "id"=>3,
                "name"=>"笔记本",
                "pid"=>1,
                "children"=>array(
                    //此处省略
                )
            ),
            &array(
                "id"=>4,
                "name"=>"台式机",
                "pid"=>1,
                "children"=>array(
                    //此处省略
                )
            ),
        )
    ),
    //其他分类省略
)

处理过程：

$tree = array();
//第一步，将分类id作为数组key,并创建children单元
foreach($categories as $category){
    $tree[$category["id"]] = $category;
    $tree[$category["id"]]["children"] = array();
}
//第二部，利用引用，将每个分类添加到父类children数组中，这样一次遍历即可形成树形结构。
foreach ($tree as $k=>$item) {
    if ($item["pid"] != 0) {
        $tree[$item["pid"]]["children"][] = &$tree[$k];
    }
}
print_r($tree);

打印结果如下：

Array
(
    [1] => Array
        (
            [id] => 1
            [name] => 电脑
            [pid] => 0
            [children] => Array
                (
                    [0] => Array
                        (
                            [id] => 3
                            [name] => 笔记本
                            [pid] => 1
                            [children] => Array
                                (
                                    [0] => Array
                                        (
                                            [id] => 7
                                            [name] => 超级本
                                            [pid] => 3
                                            [children] => Array
                                                (
                                                )
                                        )
                                    [1] => Array
                                        (
                                            [id] => 8
                                            [name] => 游戏本
                                            [pid] => 3
                                            [children] => Array
                                                (
                                                )
                                        )
                                )
                        )
                    [1] => Array
                        (
                            [id] => 4
                            [name] => 台式机
                            [pid] => 1
                            [children] => Array
                                (
                                )
                        )
                )
        )
    [2] => Array
        (
            [id] => 2
            [name] => 手机
            [pid] => 0
            [children] => Array
                (
                    [0] => Array
                        (
                            [id] => 5
                            [name] => 智能机
                            [pid] => 2
                            [children] => Array
                                (
                                )
                        )
                    [1] => Array
                        (
                            [id] => 6
                            [name] => 功能机
                            [pid] => 2
                            [children] => Array
                                (
                                )
                        )
                )
        )
    [3] => Array
        (
            [id] => 3
            [name] => 笔记本
            [pid] => 1
            [children] => Array
                (
                    [0] => Array
                        (
                            [id] => 7
                            [name] => 超级本
                            [pid] => 3
                            [children] => Array
                                (
                                )
                        )
                    [1] => Array
                        (
                            [id] => 8
                            [name] => 游戏本
                            [pid] => 3
                            [children] => Array
                                (
                                )
                        )
                )
        )
    [4] => Array
        (
            [id] => 4
            [name] => 台式机
            [pid] => 1
            [children] => Array
                (
                )
        )
    [5] => Array
        (
            [id] => 5
            [name] => 智能机
            [pid] => 2
            [children] => Array
                (
                )
        )
    [6] => Array
        (
            [id] => 6
            [name] => 功能机
            [pid] => 2
            [children] => Array
                (
                )
        )
    [7] => Array
        (
            [id] => 7
            [name] => 超级本
            [pid] => 3
            [children] => Array
                (
                )
        )
    [8] => Array
        (
            [id] => 8
            [name] => 游戏本
            [pid] => 3
            [children] => Array
                (
                )
        )
)

优点：关系清楚，修改上下级关系简单。

缺点：使用PHP处理，如果分类数量庞大，效率也会降低。