PHP 两级联动数据库生成JSON

有上图结构的数据表，欲形成如下格式的JSON数据，PHP代码该如何写？

[
    {
        "name":"电脑",
        "id":"1",
        "list":[
            {
                "name":"无法开机",
                "id":"3"
            },
            {
                "name":"无法联网",
                "id":"4"
            },
            {
                "name":"蓝屏",
                "id":"5"
            }
        ]
    },
    {
        "name":"打印机",
        "id":"2",
        "list":[
            {
                "name":"卡纸",
                "id":"6"
            },
            {
                "name":"加粉",
                "id":"7"
            }
        ]
    }
]

$sql="select id,name,superior from table where 1=1";
$requ = mysqli_query($con,$sql);
$items = array();
while($rs=mysqli_fetch_array($requ)){
    $a=array("title"=>$rs['name'],"value"=>$rs['id'],"pid"=>$rs['superior']);
    array_push($items,$a);
}

$generateTree = function ($items, $parentKey = 'pid') {
    $tree = $temp = array();
    foreach ($items as $item) {
        $temp[$item['value']] = $item;
    }
    foreach ($items as $item) {
        if (isset($temp[$item[$parentKey]])) {
            $temp[$item[$parentKey]]['list'][] = &$temp[$item['value']];
        } else {
            $tree[] = &$temp[$item['value']];
        }
        unset($temp[$item['value']][$parentKey]);
    }
    return $tree;
};

$data = json_encode($generateTree($items));
echo $data;