Sounds a little like the original developer might be relying on auto_globals, which would mean that all you'd have to do is type http://www.somefancydomain.com/index.php?current_page=#., and it'd be set for you.
For security reasons, this is not a good idea, and could be a Very Bad Idea. Perhaps you could write a quick validation function to make sure it's set, and within reasonable bounds.