<?php

require_once '../model/login_class.php';
$action = trim($_REQUEST['action']);
$controller = new login_controller();
if ($action == "login") {
    $username = trim($_REQUEST['txtUsername']);
    $password = trim($_REQUEST['txtPassword']);
    $controller->validalogin($username, $password);
} elseif ($action == "logout") {
    $controller->logout();
}
class login_controller
{
    private $login_class;
    function __construct()
    {
        $this->login_class = new login_class();
    }
    public function validalogin($username, $password)
    {
        $user_exist = $this->login_class->login($username, $password);
        echo $user_exist;
    }
    public function logout()
    {
        $this->login_class->logout();
    }
}
<?php

session_start();
require_once '../model/login_class.php';
$proceso = $_REQUEST['solicitud'];
$instancia = new login_controller($_REQUEST['txtUsername'], $_REQUEST['txtPassword'], $_REQUEST['cboSeleccionRol']);
switch ($proceso) {
    case "validalogin":
        $instancia->validalogin();
        break;
    case "logout":
        $instancia->logout();
        break;
}
class login_controller
{
    private $username;
    private $password;
    private $login;
    function __construct($username, $password)
    {
        $this->username = $username;
        $this->password = $password;
        $this->login = new login_class();
    }
    public function validalogin()
    {
        $validacion = $this->login->login($this->username, $this->password);
        if (!$validacion) {
            echo "<script>alert('El usuario y contrasena son incorrectas')</script>";
            echo '<meta http-equiv="refresh" content="0; url=../index.php" />';