classes/functions and user fields

[KrU$ty]

I was wondering what the correct usage is for a class when i want to use a field for a variable. Here is part of the class im trying to get working:

class Code
{
    private $key = $bbuserinfo[fieldx];
    private $usercode = $bbuserinfo[fieldx];
    private $charcode = $bbuserinfo[fieldx];
    private $cachedir = './xmlcache';
    private $cachetime = 60;
    private $timeformat = "Y-n-d H:i:s";
    public $debug = false;
    private $msg = array();
    private $usecache = false;

where $bbuerinfo[fieldx] refers to a user defined field. This class is held in /includes/.

What is the correct usage of the field attribute within a class? Also, how might i use this same method with scripts outside of the vbulletin structure?

Thanks in advance.