Display if Online - Display if Offline

[Osterling]

I have the following code at the start of all my php pages:

/* vBulletin Information Retrieval*/
$curdir = getcwd ();
chdir('/home/hghgfhgfh/public_html/messageboard');
require_once('/home/hghgfhgfh/public_html/messageboard/global.php');
chdir ($curdir);

/* Name and User Id of User*/
$user_id = $vbulletin->userinfo['userid'];
$user_name = $vbulletin->userinfo['username'];

This allows me on non vb pages to tell if a user is log in and if so, there username. What I am trying to do now is show if another user is online or not. Is there a script I can just plug in a userid and tell if that member is logged in or not? Thank you

Or at least tell me where it stores who is online and who isn't in the vb database