import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;
import android.widget.Toast;


public class PatLogin extends Activity {

    EditText a,b;
    String login_name,login_pass;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.pat_login);

    }

    public void patbuttonClick(View v) {
        if (v.getId() == R.id.patlogin) {
            a = (EditText) findViewById(R.id.TFpusername);
            b = (EditText) findViewById(R.id.TFppassword);
            login_name = a.getText().toString();
            login_pass = b.getText().toString();


            String method = "login";
            BackgroundTask backgroundTask = new BackgroundTask(this);
            backgroundTask.execute(method,login_name,login_pass);

            //If possible I would like to call the "Display" activity from here but only when the correct username and password is entered.
           //If it's not possible to call from here then I would like to know how to call "Display" activity from "BackgrounTask.java".





        }

    }
}

import android.app.AlertDialog;
import android.content.Context;
import android.content.Intent;
import android.os.AsyncTask;
import android.widget.Toast;
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 BackgroundTask extends AsyncTask<String,Void,String> {
    AlertDialog alertDialog;
    Context ctx;

    BackgroundTask(Context ctx)
    {
        this.ctx =ctx;
    }
    int flag=0;
    @Override
    protected void onPreExecute() {
        alertDialog = new AlertDialog.Builder(ctx).create();
        alertDialog.setTitle("Login Information....");
    }
    @Override
    protected String doInBackground(String... params) {

        String login_url = "http://10.0.2.2/mobidoc/login.php";
        String method = params[0];

            if(method.equals("login"))
        {
            String login_name = params[1];
            String login_pass = params[2];
            try {
                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 data = URLEncoder.encode("login_name","UTF-8")+"="+URLEncoder.encode(login_name,"UTF-8")+"&"+
                        URLEncoder.encode("login_pass","UTF-8")+"="+URLEncoder.encode(login_pass,"UTF-8");
                bufferedWriter.write(data);
                bufferedWriter.flush();
                bufferedWriter.close();
                outputStream.close();
                InputStream inputStream = httpURLConnection.getInputStream();
                BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(inputStream,"iso-8859-1"));
                String response = "";
                String line = "";
                while ((line = bufferedReader.readLine())!=null)
                {
                    response+= line;
                }
                bufferedReader.close();
                inputStream.close();
                httpURLConnection.disconnect();
                return response;
            } catch (MalformedURLException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return null;
    }
    @Override
    protected void onProgressUpdate(Void... values) {
        super.onProgressUpdate(values);
    }
    @Override
    protected void onPostExecute(String result) {
        if(result.equals("Registration Success..."))
        {
            Toast.makeText(ctx, result, Toast.LENGTH_LONG).show();
        }
        else
        {
            alertDialog.setMessage(result);
            alertDialog.show();
            //When I use the following 2 lines of code, It calls the Display activity even if the wrong password is entered. I need a certain condition to be applied.
            Intent myIntent = new Intent(ctx, Display.class);
            ctx.startActivity(myIntent);
        }
    }


}

<?php

require "init.php";
$username = $_POST["login_name"];
$password = $_POST["login_pass"];
$password = md5($password);
$sql_query = "select name from pat_info where username like '$username' and password like '$password';";

$result = mysqli_query($con,$sql_query);

if(mysqli_num_rows($result)>0)
{
$row = mysqli_fetch_assoc($result);

$name = $row["name"];
echo "Login Success... Welcome ".$name;}

else{
echo "Login Failed...Try Again.";
}