Php MySQL Login error Undefined index

Question

I'm newbie and have been trying to make a login page for the first time. I've been following some videos on YouTube and been exactly Copy/Pasting the codes but I get this undefined index that says my username and password haven't been defined! I've made a test user in my database but can't login. the connection to the server is successful but the login page has user and pass issues.

My Login.php code:

   <?php
require "connect.php";
$user_name =$_POST['user_name'];
$user_pass =$_POST['password'];
$mysql_qry = "select * from logintesttb where username like '$user_name' and     password '$user_pass';";
$result = mysqli_query($conn, $mysql_qry);
if(mysqli_num_rows($result) >  0){
    echo "Login was successful...!";
}
else{
    echo "Oops! There was an error";
}

?>

My Java Code:

protected String doInBackground(String... params) {
    String type = params[0];
    String login_url = "http://My IP Address/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;
}

My OnClickListener for Login Button:

public void OnLogin(View view) {
    String username = mUsername.getText().toString();
    String password = mPassword.getText().toString();
    String type = "login";
    BackgroundWorker backgroundWorker = new BackgroundWorker (this);
    backgroundWorker.execute (type, username, password);

}

please help me out if possible. thanks in advance...!