logging forum user names/id

[KT Walrus]

I'm hosting a forum.  I'd like to log the forum user name/id into  
nginx access_log.

What is the best way to do this?

Should I have forum php code set a header with this user name/id and  
somehow get this into an nginx variable I can use in the log_format  
statement?

Or, some other "better" approach?

Also, is there some way to print out all nginx conf variables that are  
visible (like phpinfo() does for php)?  I'm having a hard time with  
the English wiki documentation as names of variables seem to be in  
multiple places (some of which I can't find).

Kevin