Urgent help needed plz. How to create a 'user' folder dynamically in 'application' dir?

[eluser]zrowcrypt[/eluser]
I want to create a user folder(name = user_id) in app folder the first time user logs in?

I am not able to do that. I guess it has something to do with mkdir($path). How can I get a
$path to point to /app/users/id_12345?

I have spent a lot of time trying to figure this out. Can anyone plz help?