how? register and enter protected directory

I have Phorums running on my site. But I want people to be able to register first (username and password) then log-in (getting the username and passworddata from a MySQL database) before they can enter the directory of Phorums. And I also plan to put a logout link so that they can logout from Phorums session/folder. After logging out, they have to login again in order to enter the Phorums. I can make the registration script and enter the data into MySQL. My problem is the logging-in. Should I user cookies or sessions? Will sessions work in protecting the Phorums directory if, for example, 4 people are using one internet connection via internet sharing? Does anyone have a script that can do the above? Thanks for your help. - Andres Montiel