Problems with session data.

I thought I was using session data and variables correctly, but I am having some issues. I am using it to create a login but if I have another user on a different machine my session data gets changed to who last logged in. I had changed so it used https but changed it back and it still seems to be doing the same. Is there something maybe I missed in the php.ini file? Can it not be used this way? Do I have to uses cookies? Any help/suggestions would be appreciated. Scott Duffy