JSON malformed exception (Android Kotlin Retrofit PHP MySQL)

I am developing an app with Kotlin-PHP-MySql.

I will be appreciated if someone has an idea why I get this exception "com.google.gson.stream.MalformedJsonException: Use JsonReader.setLenient(true) to accept malformed JSON at line 1 column 1 path" when I call createUser function.

I don't get an exception when I call getUser function with Same api.

I checked these two functions in postman. JSONs are formatted properly. Begins with { "key" : "value", "key1" : "value1", .... }

This is my api:

     interface Api {
     ...
     @POST("/api/v1/users/register_user.php")
     suspend fun createUser(
         @Body userDto: UserInfoPreviewDto
     ) : QueryResponse

     @POST("/api/v1/users/get_user.php")
     suspend fun getUser(
         @Body  jsonObject : JsonObject
     ) : UserDto
    }

and my Retrofit instance:

    Retrofit.Builder()
        .baseUrl(BASE_URL)
        .addConverterFactory(GsonConverterFactory.create())
        .build()
        .create(RamonApiUser::class.java)

and my server-side php code is register_user.php file:

      header("Access-Control-Allow-Origin: *");
      header("Content-Type: application/json; charset=UTF-8");
      header("Access-Control-Allow-Methods: POST");
      header("Access-Control-Allow-Headers: Content-Type, Access-Control-Allow- 
            Headers, Authorization, X-Requested-With");
      include_once("../../config/database.php");
      include_once("../../classes/user.php");
      $db = new Database();
      $connection = $db->connect();
      $user = new User($connection);
      $query_response = new QueryResponse();
      if($_SERVER['REQUEST_METHOD'] === "POST") {
        $data = json_decode(file_get_contents("php://input"));
        $user->username =  $data->username;
        $user->password =  $data->password;
        $user->fullname =  $data->fullname;
        $user->email =  $data->email;
        if($user->register()) {
            http_response_code(200);
            $result = array( 
                "status" => "200", 
                "message" => "saved"
            );
            echo json_encode($result);
         }
         else {
            http_response_code(500);
            echo json_encode(array(
                "status" => "500",
                "message" => "Error 500"   
            ));
        }    
      }
      else {
          http_response_code(503);
          echo json_encode(array(
                                 "status" => "503",
                                 "message" => "Error 503"   
                           ));
      }

this my get_user.php file:

    header("Access-Control-Allow-Origin: *");
    header("Content-Type: application/json; charset=UTF-8");
    header("Access-Control-Allow-Methods: POST");
    header("Access-Control-Allow-Headers: Content-Type, Access-Control-Allow-Headers, 
             Authorization, X-Requested-With");
    include_once("../../config/database.php");
    include_once("../../classes/user.php");
    $db = new Database();
    $connection = $db->connect();
    $user = new User($connection);  
    if($_SERVER['REQUEST_METHOD'] === "POST") {
        $param = json_decode(file_get_contents("php://input"));
        if(!empty($param->id)) {
            $user->id = $param->id;
            $user_data = $user->getById();
            if (!empty($user_data)) {
                http_response_code(200);
                $user_data['isPublicProfile'] = (bool)$user_data['isPublicProfile']; 
                $user_data['isLastOnlineVisible'] = 
                                     (bool)$user_data['isLastOnlineVisible'];
                $user_data['isPhoneNumberVisible'] = 
                                     (bool)$user_data['isPhoneNumberVisible'];
                echo json_encode($user_data);
            }
        } 
     }
     else {
          http_response_code(503);
          echo json_encode(array(
                     "status" => "0",
                     "message" => "503 error"   
       ));
     }

getById() function in user.php :

    public function getById() {
        $sqlQuery = "SELECT * FROM ". $this->table_name ." WHERE id = ? LIMIT 0,1";
        $obj = $this->conn->prepare($sqlQuery);
        $obj->bind_param("i", $this->id);
        $obj->execute();
        $data = $obj->get_result();
        return $data->fetch_assoc();
    }

I have QueryResponse data class in the client-side with the fields status and message. user->register() returns true or false . I think the problem is with my json_encode method. I also tried this: I created a QueryReponse php class with fields status and message and encode this object like this:

    $query_response = new QueryResponse();
    $query_response->status = "200";
    $query_response->message = "registration successful";
    $echo json_encode($query_response);

it didn't help either.

source https://stackoverflow.com/questions/71242583/json-malformed-exception-android-kotlin-retrofit-php-mysql