This is my android function to read the response, I'm using retrofit 2.

    public void onClick(@org.jetbrains.annotations.NotNull final View view) {
        if (view.getId() == R.id.btnLogon) {
            managerSecurityApi.login (etUsername.getText ().toString (), etPassword.getText ().toString (), new Callback<List<ObjectUser>> () {
                @Override
                public void onResponse (@NonNull Call<List<ObjectUser>> call, @NonNull Response<List<ObjectUser>> response) {
                    List<ObjectUser> JsonList = requireNonNull(response).body ();
                    for (int i = 0; i < requireNonNull (JsonList).size (); i++)
                        objectUser = new ObjectUser (
                            JsonList.get (i).getId (),
                            JsonList.get (i).getUsername (),
                            JsonList.get (i).getGrade (),
                            JsonList.get (i).getGradename(),
                            JsonList.get (i).getDepartment (),
                            JsonList.get (i).getDepartmentname()
                        );
                    getContentResolver ().delete (ProviderUser.CONTENT_URI, null, null);
                    etUsername.setText (R.string.blank);
                    etPassword.setText (R.string.blank);
                    ContentValues values = new ContentValues ();
                    values.put (ProviderUser.name, objectUser.getUsername ());
                    values.put (ProviderUser.grade, String.valueOf (objectUser.getGrade ()));
                    values.put (ProviderUser.gradename, objectUser.getGradename ());
                    values.put (ProviderUser.department, String.valueOf (objectUser.getDepartment ()));
                    values.put (ProviderUser.departmentname, objectUser.getDepartmentname());
                    getContentResolver ().insert (ProviderUser.CONTENT_URI, values);
                    showApps ();
                }
                @Override
                public void onFailure (@NonNull Call<List<ObjectUser>> call, @NonNull Throwable t) {
                    Toast.makeText (getApplicationContext (), t.getMessage (), Toast.LENGTH_SHORT).show ();
                }
            });
        } else {
            throw new IllegalStateException("Unexpected value: " + view.getId());
        }
    }

This is my API to login. It is written in PHP, using the Slim Microframework.

        public function login(LoggingUser $loggingUser) {
            $return_value[0]['success'] = false;
            $return_value[0]['message'] = '';
            if(!$loggingUser->username){
                $return_value[0]['message'] = 'username field  required in Security::login';
            } else {
                if (!$loggingUser->password) {
                    $return_value[0]['message'] = 'password field required in Security::login';
                } else {
                    $return_value[0]['success'] = true;
                    $return_value[0]['message'] = 'User successfully retrieved';
                    $return_value[0]['data'] = $this->repository->login($loggingUser);
                }
            }
            return $return_value;
        }

This is what postman return when calling the api from my website.
[
    {
        "success": true,
        "message": "User successfully retrieved",
        "data": {
            "username": "admin",
            "grade": "4",
            "grade_text": "Administrator",
            "department": "5",
            "department_text": "Operations"
        }
    }
]

The LogCat in android studio says it fails a NON NULL constraint when inserting the data, which is to be expected it the data is null.