[akanevsky]

What do you mean it treats them as constants?

A string must be wrapped in single or double quotes.
When you call is_browser(ie), it looks for a constant IE. Because PHP is a very non-restrictive language, it then treats ie as a string, even though it is not; it will only display an error if E_NOTICE is enabled. But since vBulletin error reporting is E_ALL ^ E_NOTICE, you don't see the error.

The right way would be using is_browser('ie').

Also, how would you suggest that I store the browsers then??

I would store browser information in the session table, because there it is temporary and deleted automatically when the user leaves the board.