Android从PHP读取JsonObject作为字符串

Question

I want to get my json result from my php and display it in android, I tried many solutions and nothing is working for me. In android side I used JSONParser and always I get json = null in this ligne JSONObject json=jParser.getJSONFromUrl(url); with this error: Error parsing data org.json.JSONException: Value n of type java.lang.String cannot be converted to JSONObject

I will post my php code because my android code I am sure it is working since I tested it with another php code using codeigniter framework.

function get_user($id = 0) {
  global $connection;
  $query = "SELECT * FROM users";
  if ($id != 0) {
    $query. = "WHERE id =".$id. "LIMIT 1";
  }
  $response = array();
  $result = mysqli_query($connection, $query);
  while ($row = mysqli_fetch_array($result)) {
    array_push($response, array("id" => $row['id'], "username" => $row['username']));
  }
  header('Content-Type: application/json');
  echo json_encode(array('user' => $response));
  mysli_close($connection);
}

the result while executing my url is :

{
  "user": [
    {
        "id": "1",
        "username": "myusername"

    }
  ]
}