public function test()
{
    // 初始数据
    $items = array(
        array('id' => 1, 'pid' => 0, 'name' => '福建省'),
        array('id' => 2, 'pid' => 0, 'name' => '四川省'),
        array('id' => 3, 'pid' => 1, 'name' => '福州市'),
        array('id' => 4, 'pid' => 2, 'name' => '成都市'),
        array('id' => 5, 'pid' => 2, 'name' => '乐山市'),
        array('id' => 6, 'pid' => 4, 'name' => '成华区'),
        array('id' => 7, 'pid' => 4, 'name' => '龙泉驿区'),
        array('id' => 8, 'pid' => 6, 'name' => '崔家店路'),
        array('id' => 9, 'pid' => 7, 'name' => '龙都南路'),
        array('id' => 10, 'pid' => 8, 'name' => 'A店铺'),
        array('id' => 11, 'pid' => 9, 'name' => 'B店铺'),
        array('id' => 12, 'pid' => 8, 'name' => 'C店铺'),
        array('id' => 13, 'pid' => 1, 'name' => '泉州市'),
        array('id' => 14, 'pid' => 13, 'name' => '南安县'),
        array('id' => 15, 'pid' => 13, 'name' => '惠安县'),
        array('id' => 16, 'pid' => 14, 'name' => 'A镇'),
        array('id' => 17, 'pid' => 14, 'name' => 'B镇'),
        array('id' => 18, 'pid' => 16, 'name' => 'A村'),
        array('id' => 19, 'pid' => 16, 'name' => 'B村'),
    );
 
    // 根据初始数据，生成一个以 id 为 key/下标 的数组，方便根据 pid 判断是否存在父级元素。
    $items = array_column($items,null,'id');
 
    //使用 php 的 & 引用特性，遍历一次循环即可生成无限级分类树。（其他高级语言中也有类似的特性，诸如 C++ 的指针和 JAVA 的引用）
    $tree = [];
    foreach ($items as $item) {
        $id = $item['id'];
        $pid = $item['pid'];
 
        if (isset($items[$pid]))
            $items[$pid]['children'][] = &$items[$id];
        else
            $tree[] = &$items[$id];
    }
 
    $this->success('ok',$tree);
}

{
  "code": 1,
  "msg": "ok",
  "data": [
    {
      "id": 1,
      "pid": 0,
      "name": "福建省",
      "children": [
        {
          "id": 3,
          "pid": 1,
          "name": "福州市"
        },
        {
          "id": 13,
          "pid": 1,
          "name": "泉州市",
          "children": [
            {
              "id": 14,
              "pid": 13,
              "name": "南安县",
              "children": [
                {
                  "id": 16,
                  "pid": 14,
                  "name": "A镇",
                  "children": [
                    {
                      "id": 18,
                      "pid": 16,
                      "name": "A村"
                    },
                    {
                      "id": 19,
                      "pid": 16,
                      "name": "B村"
                    }
                  ]
                },
                {
                  "id": 17,
                  "pid": 14,
                  "name": "B镇"
                }
              ]
            },
            {
              "id": 15,
              "pid": 13,
              "name": "惠安县"
            }
          ]
        }
      ]
    },
    {
      "id": 2,
      "pid": 0,
      "name": "四川省",
      "children": [
        {
          "id": 4,
          "pid": 2,
          "name": "成都市",
          "children": [
            {
              "id": 6,
              "pid": 4,
              "name": "成华区",
              "children": [
                {
                  "id": 8,
                  "pid": 6,
                  "name": "崔家店路",
                  "children": [
                    {
                      "id": 10,
                      "pid": 8,
                      "name": "A店铺"
                    },
                    {
                      "id": 12,
                      "pid": 8,
                      "name": "C店铺"
                    }
                  ]
                }
              ]
            },
            {
              "id": 7,
              "pid": 4,
              "name": "龙泉驿区",
              "children": [
                {
                  "id": 9,
                  "pid": 7,
                  "name": "龙都南路",
                  "children": [
                    {
                      "id": 11,
                      "pid": 9,
                      "name": "B店铺"
                    }
                  ]
                }
              ]
            }
          ]
        },
        {
          "id": 5,
          "pid": 2,
          "name": "乐山市"
        }
      ]
    }
  ]