PHP 嵌套类别和子类别 JSON

我想为我的应用程序获取带有类别和子类别的嵌套 JSON，这是我的代码

// get all items from myorder table
$result = mysql_query("SELECT co.country, cl.city_code, cl.city_name
                       FROM country as co, city_list as cl
                       WHERE co.id_country = cl.id_country") or die(mysql_error());
$response["location"] = array();
while ($row = mysql_fetch_assoc($result)) {
    $categories['Cities'][] = array(
            'code'         => $row['city_code'],
            'city'         => $row['city_name'],
    );
}
foreach ($categories as $key => $value) {
    $category_data[] = array(
        'category'      => $key,
        'category_list' => $value,
    );
}
array_push($response["location"], $categories);
// echoing JSON response
echo json_encode($response);

这是结果：

 {
location: [
    {
        USA: {
            Country: "USA"
            Cities: [
                {
                    code: "AL",
                    city: "ALABAMA"
                },
                {
                    code: "AR",
                    city: "Arkansas"
                },
                {
                    code: "DC",
                    city: "Distric of Columbia"
                },
                {
                    code: "LA",
                    city: "Louisiana"
                }
                ]
            }
        }
    ]
}

我想得到这个格式：

{
location: [
    Country: "USA"
    Cities: [
        {
            code: "AL",
            city: "ALABAMA"
        },
        {
            code: "AR",
            city: "Arkansas"
        },
        {
            code: "DC",
            city: "Distric of Columbia"
        },
        {
            code: "LA",
            city: "Louisiana"
        },

    ]
}

我怎样才能做到这一点？有什么参考资料可以参考吗？有什么帮助吗？