[PHP] World Trade Centre Hit By terrorists, also pentagon

[PHP] User Authentication / only 1 login per user at any time

Hi, With most of my scripts, when I am about to write something, I have it in my head on how to do it exactly.. But with only 1 login per user at anytime, I havent found any good ways to do it, All I can think of is to set a mysql field called ³logged_in² to ³1² or ³0² if logged in or not, but