private void registerUser() {
    JSONObject postObject = new JSONObject();
    RequestQueue queue =  Volley.newRequestQueue(this);
    JSONObject historyObject = new JSONObject();

    String url ="http://helpinghandbd.org/app/index.php";
    try {
        //historyObject.put("id","1");
        historyObject.put("email","1234");
        historyObject.put("password","1234");
        postObject.put("user",historyObject);
    } catch (JSONException e) {
        e.printStackTrace();
    }

    Log.e("LoginActivityJsonObject",""+postObject);
    JsonObjectRequest objRequest = new JsonObjectRequest(Request.Method.POST, url,postObject,
            new Response.Listener<JSONObject>() {
                @Override
                public void onResponse(JSONObject response) {
                    Log.e("LoginActivity","OnResponse: "+response);
                    Toast.makeText(LoginActivity.this, String.valueOf(response), Toast.LENGTH_LONG).show();
                }
            }, new Response.ErrorListener() {
        @Override
        public void onErrorResponse(VolleyError error) {
            Log.e("OnError", String.valueOf(error.getMessage()));
        }
    });

    queue.add(objRequest);

}

{ 'user':{
           'email':'1234',
           'password':'1234'
        }
 }

<?php

$data = file_get_contents("php://input");
//echo $data; -> //{ 'user':{'email':'1234','password':'1234'}};
$decode = json_decode($data,true);
$email = $decode->user['email'];
$password = $decode->user['passowrd'];

$servername = "localhost";
$username = "helpinghandbd_app";
$password = "Demopass000";
$dbname = "helpinghandbd_app";

// Create connection
$conn = new mysqli($servername, $username, $password, $dbname);
// Check connection
if ($conn->connect_error) {
    die("Connection failed: " . $conn->connect_error);
}

//$data = file_get_contents("php://input");
//{ 'user':{'email':'1234','password':'1234'}};



$sql = "INSERT INTO users (id,email,password)
VALUES (null, '$email', '$password')";

if ($conn->query($sql) === TRUE) {
    echo "New record created successfully";
} else {
    echo "Error: " . $sql . "<br>" . $conn->error;
}



$conn->close();