I would like to convert my PHP script to JSON to return a specific structure with parent and child. I searched some similar questions here, but unfortuntly i didn’t find a correct way to solve my doubt:

1. How to create json array php with childs
2. php json parse access child values
3. Php echo json array with children
4. How to select JSON (dynamic) Parent and Child in PHP?

I have a database table that call tbl_countries that’s similar to the structure below:

id | countryID | country | city
-----------------------------------------
 1 |     1     | Greece  | Athens
 2 |     1     | Greece  | Thessalonica
 3 |     2     | Italy   | Rome
 4 |     2     | Italy   | Milan

What i would like to do is convert my php script to return a json structure as below:

    [{
      "id": 1,
      "text": "Greece",
      "children": [{
        "id": "Athens",
        "text": "Athens"
      }, {
        "id": "Thessalonica",
        "text": "Thessalonica"
      }]
    }, {
      "id": 2,
      "text": "Italy",
      "children": [{
        "id": "Milan",
        "text": "Milan"
      }, {
        "id": "Rome",
        "text": "Rome"
      }]
    }]

Below is the php script that i’m trying to convert like json structure above. This script, when executed, return information correctly, but not like the structure that i would like to has:

include 'config.php';

$stmt = $mysqli -> prepare('SELECT countryID, country, city FROM tbl_countries'); 
 
$stmt -> execute(); 
$stmt -> store_result(); 
$stmt -> bind_result($countryID, $country, $city);

$json = array();

while ($stmt -> fetch()) {

$json[] = array("id" => $countryID, "text" => $country, "children" => [array("id" => $city, "text" => $city)]);

}

header('Content-Type: application/json');
print_r(json_encode($json, JSON_UNESCAPED_UNICODE));

In this case how can i improve my php script to return a similar json structure above?