Session contamination?

Hello All, I'm using a single development server to host multiple client projects, many of which require session management. I've noticed that sometimes when I test these various web apps (which are simply in separate sub directories) I get session leakage where logging in and establishing a session on one app allows me access to (automatically logs me in) to other app(s) on the same server. Or sometimes a session variable will be set across all the apps, like $_SESSION['username']. Is this due to the fact that sessions are established between client browsers and servers, regardless of directory/sub directory? What is the best way to avoid/prevent this problem? Should I be using specific Session ID's or Session names? Thanks for any help, - Ben