package issa.example.com.mysqldatabase;


import android.content.Intent;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

import com.kosalgeek.asynctask.AsyncResponse;
import com.kosalgeek.asynctask.PostResponseAsyncTask;

import java.util.HashMap;

public class MainActivity extends AppCompatActivity implements AsyncResponse{

    EditText etUsername, etPassword;
    Button btnLogin;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        etUsername = (EditText)findViewById(R.id.etUsername);
        etPassword = (EditText)findViewById(R.id.etPassword);
        btnLogin = (Button)findViewById(R.id.btnLogin);

        btnLogin.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                HashMap postData = new HashMap();
                postData.put("btnLogin", "Login");
                postData.put("mobile", "android");
                postData.put("user_name", etUsername.getText().toString());
                postData.put("pass_word", etPassword.getText().toString() );

                PostResponseAsyncTask loginTask =
                        new PostResponseAsyncTask(MainActivity.this, postData);
                loginTask.execute("http://mchucha.com/android/logini.php");
            }
        });
    }

    @Override
    public void processFinish(String output) {
        if(output.equals("success")){
            Intent intent = new Intent(MainActivity.this, Activity2.class);
            Toast.makeText(this, "Login Successfully",
                    Toast.LENGTH_LONG).show();
        } else {
            Toast.makeText(MainActivity.this, "Login Failed", Toast.LENGTH_SHORT).show();
        }

    }
}

PHP script text
<?php
$servername = "localhost";
$username = "mchuchac_user";
$password = "123";
$dname = "mchuchac_db";

$conn = mysqli_connect($servername, $username, $password, $dname);
    if (!$conn){
        die ("connection failed: " .mysqli_connect_error());
    }

    else {
        echo " connection success";
    }

?>

<?PHP
    include_once("connection.php");
    if( isset($_POST['user_name']) && isset($_POST['pass_word']) ) {
        $username = $_POST['user_name'];
        $password = $_POST['pass_word'];

        $query = "SELECT username, password FROM tbl_client ".
        " WHERE username = '$username' AND password = '$password'";

        $result = mysqli_query($conn, $query);

        if($result->num_rows > 0){
            if(isset($_POST['mobile']) && $_POST['mobile'] == "android"){
                echo "success";
                exit;
            }
            echo " Login success";
        } else{
            echo "Login Failed <br/>";
        }
    }
?>