Full view of active sessions

Hello. Is there any facility in PHP's session management library which allows one to have a full view of the active sessions? In other words, is it possible to check the existance and value of a given session variable in any of the active sessions? My goal is to make sure a user's access information is not used more than once to log into the system. So, when a user submits the authentication data, I would like to see if that login has already been used, i.e., there is a session variable in some session storing the login name and which contains the same value. At present, I can't think of another way of doing it other than directly accessing the session files on /tmp (assuming session data is being stored in files). But I don't like that idea quite so much. A cleaner solution which I have also considered is to have a column in the users table which indicates his login state. Before I go for the latter approach, I would just like to learn from you whether what I asked is possible. Thanks in advance, -- Ney André de Mello Zunino