In this hour, you looked at different ways of saving state in a stateless protocol. All methods use some combination of cookies and query strings, sometimes combined with the use of files or databases. These approaches all have their benefits and problems.
You learned that a cookie alone is not intrinsically reliable and cannot store much information. On the other hand, it can persist over a long period of time. Approaches that write information to a file or database involve some cost to speed and might become a problem on a popular site. Nonetheless, a simple ID can unlock large amounts of data stored on disk. To ensure that as many users as possible get the benefit of your session-enabled environment, you can use the SID constant to pass a session ID to the server as part of a query string.
With regards to sessions themselves, you learned how to initiate or resume a session with session_start(). When in a session, you learned how to add variables to the $_SESSION superglobal, check that they exist, unset them if you want, and destroy the entire session.