asp like server variabels in php?

i have an login screen and i need to check if people are logged in on other pages. i also need to know in what usergroup they belong because that gives them rights to acces or not to acces certain parts of my site. i could do this through a link but that as insecure as he*l i could do this by using cookies but the cookie needs to be deleted when the browser is closed because the machines are used by several employees each day. i could put up a database and write in the database if someone is logged on or not, but that not secure. because if user1 is logged on then someone using username user1 could just pass on at the password gate. in asp i used session_variables to get this done but im just a newbee on php so please tell me if there's a sollution for this