updating fields

[wxdqz]

hi, got a small question here. Say I want to know how many people are in my forum so when they log into the forum I would update the active field in my table to 1 or yes whichever, but know they leave the forum, how do i let mysql know so it can update the table to 2 or no??? Should I use a cookie to do this?? Or is there a simplier way to do this???

Thanks,

Chris~