Parse bbcode in any php file

[liquidx]

I'm trying to parse BBCode in a PHP script, but it gives errors when its not in the same directory as the forums..

I chdir("forums/"); before including global.php and it eliminates the file not found errors from the includes, but it still doesn't work.. I get:
Fatal error: Call to a member function on a non-object in /www/forums/includes/functions.php on line 899

When I use the same code from within the forums directory, it works just fine..
Why?