递归实现无限极分类

 2,
      'pid' => 1,
      'title' => '济南',
    ],
    [
        'id' => 3,
        'pid' => 1,
        'title' => '滨州',
    ],
    [
        'id' => 4,
        'pid' => 2,
        'title' => '历城区',
    ],
    [
        'id' => 5,
        'pid' => 2,
        'title' => '历下区',
    ],
    [
        'id' => 6,
        'pid' => 3,
        'title' => '沾化',
    ],
    [
        'id' => 7,
        'pid' => 3,
        'title' => '无棣',
    ],
    [
        'id' => 1,
        'pid' => 0,
        'title' => '山东',
    ],
    [
        'id' => 8,
        'pid' => 0,
        'title' => '北京',
    ],
    [
        'id' => 9,
        'pid' => 8,
        'title' => '朝阳区',
    ],
];
$res = [];

function get_attr($arr,$pid){
    $tree = array();                                //每次都声明一个新数组用来放子元素
    foreach($arr as $v){
        if($v['pid'] == $pid){                      //匹配子记录
            $v['child'] = get_attr($arr,$v['id']); //递归获取子记录
            if($v['child'] == null){
                unset($v['child']);             //如果子元素为空则unset()进行删除，说明已经到该分支的最后一个元素了（可选）
            }
            $tree[] = $v;                           //将记录存入新数组
        }
    }
    return $tree;                                  //返回新数组
}

$res = get_attr($arr, 0);

var_dump($res);