I'm using controller/method as callbacks to application routes.

E.g.:

```
$app->get('/user/login', array('User', 'login'));

```

Here is the sample `User` class:

```
Class User {
  public function login() {
    print 'Login page.';
  }
}

```

How can I make the `$app` variable available automatically in all classes?  

I know I can get it using `getInstance()` method, but I need to call it manually every time.

Thanks!