Validation trial test

//login or validation
            # code...
            $id = isset($_REQUEST['id']);
            $user = $_REQUEST['username'];
            $pass = md5($_REQUEST['password']);

            //query data
            //$query = ("SELECT * from user");

            //query2
            $query = ("SELECT * FROM user WHERE id='$id' AND username='$user' AND password='$pass'");

            $data = array();
            $result = $mysqli->query($query);
            while ($row = mysqli_fetch_row($result)){

                $data[]= array(
                    'status' => 'Success',
                    "id" => $row[0],
                    "username" => $row[1],
                    "password" => $row[2],

                    );
            }

            echo json_encode($data);