From PHPBuilder.com today, there's a new tutorial that looks at one of the more handy features built into PHP, one that makes personalized sites a whole lot easier - sessions - and their management.
In a nutshell, sessions are the way that we "maintain state" from one page to the next, that is, how we identify specific users across multiple page requests.
When the user comes to the site, we need to be able to give them a unique identifier (a session id) that will allow us to know the difference between each user on the site. So, when the user logs in, and then modifies their choices for foo.php, we know whose profile to save those changes to.
To help prevent this, they give some examples of securing your sessions. Then, to illustrate all of the above points, they provide some simple code validate the session information that's been set.