PHP, MySQL and Volley

import android.content.Context;
import android.content.Intent;
import android.content.SharedPreferences;
import android.support.v7.app.AlertDialog;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.support.v7.widget.AppCompatButton;
import android.view.Menu;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

import com.android.volley.AuthFailureError;
import com.android.volley.Request;
import com.android.volley.RequestQueue;
import com.android.volley.Response;
import com.android.volley.VolleyError;
import com.android.volley.toolbox.StringRequest;
import com.android.volley.toolbox.Volley;

import java.util.HashMap;
import java.util.Map;

public class LoginActivity extends AppCompatActivity implements View.OnClickListener {

    private EditText etUsername, etPassword;
    private Button bLogin;
    private TextView tvRegisterLink;

    private boolean loggedIn = false;

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

        etUsername = (EditText) findViewById(R.id.etUsername);
        etPassword = (EditText) findViewById(R.id.etPassword);
        tvRegisterLink = (TextView) findViewById(R.id.tvRegisterLink);
        tvRegisterLink.setOnClickListener(this);
        bLogin = (Button) findViewById(R.id.bSignIn);
        bLogin.setOnClickListener(this);
    }

    @Override
    protected void onResume() {
        super.onResume();
        SharedPreferences sharedPreferences = getSharedPreferences(Config.SHARED_PREF_NAME, Context.MODE_PRIVATE);
        loggedIn = sharedPreferences.getBoolean(Config.LOGGEDIN_SHARED_PREF, false);

        //wenn login true ist
        if(loggedIn){
            Intent intent = new Intent(LoginActivity.this, UserAreaActivity.class);
            startActivity(intent);
        }
    }

    private void login(){
        final String username = etUsername.getText().toString().trim();
        final String password = etPassword.getText().toString().trim();

        StringRequest stringRequest = new StringRequest(Request.Method.POST, Config.LOGIN_URL,
                new Response.Listener<String>() {
                    @Override
                    public void onResponse(String response) {
                        //if success from server
                        if(response.equalsIgnoreCase(Config.LOGIN_SUCCESS)){
                            SharedPreferences sharedPreferences = LoginActivity.this.getSharedPreferences(Config.SHARED_PREF_NAME, Context.MODE_PRIVATE);
                            SharedPreferences.Editor editor = sharedPreferences.edit();
                            editor.putBoolean(Config.LOGGEDIN_SHARED_PREF, true);
                            editor.putString(Config.USERNAME_SHARED_PREF, username);
                            editor.commit();

                            Intent intent = new Intent(LoginActivity.this, UserAreaActivity.class);
                            startActivity(intent);
                        }else{
                            //if not success from server
                            Toast.makeText(LoginActivity.this, "Invalid username or password", Toast.LENGTH_LONG).show();
                        }
                    }
                },
                new Response.ErrorListener() {
                    @Override
                    public void onErrorResponse(VolleyError error) {
                        //Error
                    }
                }){
            @Override
            protected Map<String, String> getParams() throws AuthFailureError {
                Map<String,String> params = new HashMap<>();
                params.put(Config.KEY_USERNAME, username);
                params.put(Config.KEY_PASSWORD, password);
                return params;
            }
        };

        RequestQueue requestQueue = Volley.newRequestQueue(this);
        requestQueue.add(stringRequest);
    }


    @Override
    public void onClick(View v) {
        switch (v.getId()) {
            case R.id.tvRegisterLink:
                //  Intent registerIntent = new Intent(LoginActivity.this, RegisterActivity.class);
                // LoginActivity.this.startActivity(registerIntent);
                break;
            case R.id.bSignIn:
                login();
                break;
        }
    }
}

<?php
define("DB_HOST", "localhost");
define("DB_USER", "root");
define("DB_PASSWORD", "123");
define("DB_DATABASE", "dbserver");

$con = mysqli_connect(DB_HOST, DB_USER, DB_PASSWORD, DB_DATABASE);

if(!$con) {
    die("keine Verbindung".mysql_error());
}
echo "Erfolgreich verbunden"
?>

<?php     
    if($_SERVER['REQUEST_METHOD']=='POST'){
        $username = $_POST['username'];
        $password = $_POST['password'];

        $sql = "SELECT * FROM users WHERE username='$username' AND password='$password'";
         
        require_once('loginregister/Config.php');

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

        $check = mysqli_fetch_array($result);

        if(isset($check)){
            echo "success";
        }else{
            echo "failure";
        }
        mysqli_close($con);
    }

response.equalsIgnoreCase(Config.LOGIN_SUCCESS)

echo "Erfolgreich verbunden"

new Response.ErrorListener() {
    [USER=3517]@override[/USER]
    public void onErrorResponse(VolleyError error) {
        Toast.makeText(RegisterActivity.this, error.toString(), Toast.LENGTH_LONG).show();
    }
})