如何将数组中的字符串转换为数组并将它们组合在一起？

I have an array which look like this :

Result 1:

{
 "error_code": 0,
 "error_message": "",
 "return_data": {
    "items": [
        {
            "id": 462,
            "users": "1,36,38"
        },
        {
            "id": 462,
            "users": "1,4"
        },...... //same for 20 result
    ]
 }
}

I want the users to convert to an array,and separate by the comma,so the whole result will look like this :

The result I want:

{
    "error_code": 0,
    "error_message": "",
    "return_data": {
        "items": [
            {
                "id": 462,
                "users": [
                    {
                        "user_id": 1
                    },
                    {
                        "user_id": 36
                    },
                    {
                        "user_id": 38
                    }
                ],
            }.. //other result
        ]
    }
}

Here is what I try :

$res = "array the get the Result 1";
$items = //"I get the items array"

foreach ($items as $key => $item) {
    $usersArray = array(); //create a new Array

    //spilt the string to array separate with ","
    $usersIdArray = explode(',', $items['users']);

    //assign the "user_id" key to each value 
    foreach ($userIdArray as $key => $user) {

        $usersArray['user_id'] = $user;

    }
    //assign the result back to $res
    $res['return_data']['items']['users'] = $usersArray;
}

After I assign the array to $res with this line of code $res['return_data']['items']['users'] = $usersArray; ,my result become like below,I state the problem inside the code :

{
"error_code": 0,
"error_message": "",
"return_data": {
    "items": {
        "0":{ <-- // here suddenly appear a number for each result 
            "id": 462,
            "users": "1,36,38" //here nothing change (I want the array appear here)
        },
        "1":{
            "id": 462,
            "users": "1,36,38"
        },
        "2":{
            "id": 462,
            "users": "1,36,38"
        },
        "users": { //the array appear here but not the position I want..and it only appears 1 time.
            "user_id": "38"
        }

    }
  }
}

So my question is,how can I convert a String in an array to an array,assign value to the key,and make it inside the array?

Somebody please help..Thanks

Like the comments above, you could have done it the first time so you wouldn't need another structure conversion, but anyway, your code is already there a bit, its just you need to create another nesting since you want another level:

So you need another level here:

"users": [
    {
        "user_id": 1
    },
    {
        "user_id": 36
    },
    {
        "user_id": 38
    }
],

So in the code, just add []. This translates into:

foreach ($items as $key => $item) {
    $usersArray['id'] = $item['id'];
    $usersArray['users'] = array(); //create a new Array
    $usersIdArray = explode(',', $item['users']);
    foreach ($usersIdArray as $key => $user) {
        $usersArray['users'][] = array('user_id' => $user); // push each batch of key pair "user_id" key and value "each exploded id"
        // another level     ^
    }
    $res['return_data']['items'][] = $usersArray;
}

Here's the fiddle to check it out.