How to force user login only one device at a time ?

[rockfight]

Thank you Brandin for your reply,

Actually what I wanted was if a user tries to login from a device but is already logged in from another device earlier, let him log in from this device but log him out from all other devices. For the solution I thought of the way below but I am not sure if its possible or not. 

I may need to one of the following custom script but not sure which


1. not_logged_in.php // I may not get a handle to User class here
2. custom_login_script.php // I get a handle to User class here

I checked logout.php and it uses 

Code:

$user->logout();

When I checked "User.php" class I found it uses DB to log him out

PHP Code:

public function logout(){
        $this->_db->query("DELETE FROM users_session WHERE user_id = ? AND uagent = ?",array($this->data()->id,Session::uagent_no_version()));

        Session::delete($this->_sessionName);
        Cookie::delete($this->_cookieName);
        session_unset();
        session_destroy();
    } 


I am not very good at PHP but figured out I need to somehow combine these and use it in one of the two custom login script to get my result. 
Hope to get right direction from you.
Thank you.