Getting Error java.lang.String cannot be converted to JSONObject

Question

I have successfully send String Request but while trying with JSON Object request it show String cannot be converted to JSONObject.

java.lang.String cannot be converted to JSONObject

Java

JsonObjectRequest jsonObjectRequest = new JsonObjectRequest(Request.Method.POST, url, null,
                new Response.Listener<JSONObject>() {
                    @Override
                    public void onResponse(JSONObject response) {
                        _response.setText(response.toString());

                    }
                },
                new Response.ErrorListener() {
                    @Override
                    public void onErrorResponse(VolleyError error) {
                        _response.setText(error.getMessage().toString());
                        System.out.println(error.getMessage().toString());

                    }
                }) {
            //adding parameters to the request
            @Override
            protected Map<String, String> getParams() throws AuthFailureError {
                Map<String, String> params = new HashMap<>();
                params.put("name", _name.getText().toString());
                params.put("email", _email.getText().toString());
                return params;
            }
        };
        MySingleton.getInstance(getApplicationContext()).addToRequestQueue(jsonObjectRequest);

Php

<?php
class User {
    public $name = "";
    public $email  = "";
}

$user = new User();
$user->name = trim($_POST['name']);
$user->email  = trim($_POST['email']);

json_encode($user);
?>