не удается войти, используя базу данных phpmyadmin mysql и андроид студию

Question

Я использую android studio 3.5 и хочу создать форму входа, используя PHP. Я следовал за http://www.codebind.com/android-tutorials-and-examples/android-mysql-database-tutorial-android-login-php-mysql/, но не смог проверить значения, присутствующие в базе данных MySQL. Он подключается к базе данных, но при нажатии кнопки «Отправить» выдает следующую ошибку:

«Неопределенный индекс: имя пользователя в C: \ wamp64 \ www \ login.php в строке 3».

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <EditText
        android:id="@+id/etUserName"
        android:layout_width="214dp"
        android:layout_height="54dp"
        android:layout_alignParentEnd="true"
        android:layout_alignParentBottom="true"
        android:layout_marginEnd="96dp"
        android:layout_marginBottom="477dp"
        android:autofillHints=""
        android:hint="@string/insert_username"
        android:inputType="textPersonName"
        android:text="@string/textview" />

<Button
    android:id="@+id/btnLogin"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentEnd="true"
    android:layout_alignParentBottom="true"
    android:layout_marginEnd="163dp"
    android:layout_marginBottom="326dp"
    android:text="@string/submit"
    android:onClick="OnLogin"/>

<EditText
    android:id="@+id/etPassword"
    android:layout_width="212dp"
    android:layout_height="49dp"
    android:layout_alignParentEnd="true"
    android:layout_alignParentBottom="true"
    android:layout_marginEnd="99dp"
    android:layout_marginBottom="400dp"
    android:autofillHints=""
    android:hint="@string/insert_password"
    android:inputType="textPassword"
    android:text="@string/textview2" />

MainActivity.java

package com.example.myapplication;

import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.EditText;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {
    EditText UsernameEt, PasswordEt;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        UsernameEt = (EditText)findViewById(R.id.etUserName);
        PasswordEt = (EditText)findViewById(R.id.etPassword);
    }
    public void OnLogin(View view) {
        String username = UsernameEt.getText().toString();
        String password = PasswordEt.getText().toString();
        String type = "login";
        BackgroundWorker backgroundWorker = new BackgroundWorker(this);
        backgroundWorker.execute(type, username, password);
    }
}

BackgroundWorker class

 package com.example.myapplication;

    import android.app.AlertDialog;
    import android.content.Context;
    import android.os.AsyncTask;

    import java.io.BufferedReader;
    import java.io.BufferedWriter;
    import java.io.IOException;
    import java.io.InputStream;
    import java.io.InputStreamReader;
    import java.io.OutputStream;
    import java.io.OutputStreamWriter;
    import java.net.HttpURLConnection;
    import java.net.MalformedURLException;
    import java.net.URL;
    import java.net.URLEncoder;

    public class BackgroundWorker extends AsyncTask<String,Void,String> {
        Context context;
        AlertDialog alertDialog;
        BackgroundWorker (Context ctx) {
            context = ctx;
        }
        @Override
        protected String doInBackground(String... params) {
            String type = params[0];
            String login_url = "http://192.168.16.34/login.php";
            if(type.equals("login")) {
                try {
                    String user_name = params[1];
                    String password = params[2];
                    URL url = new URL(login_url);
                    HttpURLConnection httpURLConnection = (HttpURLConnection)url.openConnection();
                    httpURLConnection.setRequestMethod("POST");
                    httpURLConnection.setDoOutput(true);
                    httpURLConnection.setDoInput(true);
                    OutputStream outputStream = httpURLConnection.getOutputStream();
                    BufferedWriter bufferedWriter = new BufferedWriter(new OutputStreamWriter(outputStream, "UTF-8"));
                    String post_data = URLEncoder.encode("user_name","UTF-8")+"="+URLEncoder.encode(user_name,"UTF-8")+"&"+URLEncoder.encode("password","UTF-8")+"="+URLEncoder.encode(password,"UTF-8");
                    bufferedWriter.write(post_data);
                    bufferedWriter.flush();
                    bufferedWriter.close();
                    outputStream.close();
                    InputStream inputStream = httpURLConnection.getInputStream();
                    BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(inputStream,"iso-8859-1"));
                    String result="";
                    String line="";
                    while((line = bufferedReader.readLine())!= null) {
                        result += line;
                    }
                    bufferedReader.close();
                    inputStream.close();
                    httpURLConnection.disconnect();
                    return result;
                } catch (MalformedURLException e) {
                    e.printStackTrace();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return null;
        }

        @Override
        protected void onPreExecute() {
            alertDialog = new AlertDialog.Builder(context).create();
            alertDialog.setTitle("Login Status");
        }

        @Override
        protected void onPostExecute(String result) {
            alertDialog.setMessage(result);
            alertDialog.show();
        }

        @Override
        protected void onProgressUpdate(Void... values) {
            super.onProgressUpdate(values);
        }
    }

МОИ ФАЙЛЫ PHP - conn.php

<?php
$db_name="veterinary";
$mysql_username="root";
$mysql_password="";
$server_name="localhost";
$conn = mysqli_connect($server_name,$mysql_username,$mysql_password,$db_name);
?>

login.php -----

<?php
require "conn.php";
$user_name = $_POST["user_name"];
$user_pass = $_POST["password"];
$mysql_qry = "select * from student where username like '$user_name' and password like '$user_pass';";
$result = mysqli_query($conn,$mysql_qry);
if(mysqli_num_rows($result)>0){
    echo"login success";
}
else {
    echo"login failed";
}
?>