//Link Routing und jeweiligen Controller zurück geben
        switch ($path) {
            case '/':
                return new FrontPageController;
            case '/login':
                return new LoginPageController;
            case '/register':
                return new RegisterController;
            default:
                echo 'error';
        }

<form action="yourPhpFile.php" method="post">
 <p>Username: <input type="text" name="username" /></p>
 <p>Password: <input type="password" name="password" /></p>
 <p><input type="submit" /></p>
</form>