php输出json,需要嵌套数组和对象问题

https://segmentfault.com/q/1010000009985295

        $tmp = [];
            $tmp['id'] = 'aaa';
            $tmp['name'] = 'bbb';
            $tmp['vvvv'] = [
                'www'=>1,
                'ffff'=>2
            ];

            echo (json_encode($tmp));

输出是

{
"id": "aaa",
"name": "bbb",
"vvvv": {
    "www": 1,
    "ffff": 2
}
}

我想问一下如何将输出改为

{
"id": "aaa",
"name": "bbb",
"vvvv": [
    {"www": 1},
    {"ffff": 2}
]
}

我测试过这样是可以的,就是不太好用:

 

    $tmp['vvvv'] = [
                0=>(object)['www'=>1],
                1=>(object)['fff'=>2]
            ];

将$tmp['vvvv']里的元素换成数组就行了，不需要转化为object，如下

 

            $tmp = [];
            $tmp['id'] = 'aaa';
            $tmp['name'] = 'bbb';
            $tmp['vvvv'] = [
                ['www'=>1],
                ['ffff'=>2]
            ];