My web application uses sessions to store information about the user once they\'ve logged in, and to maintain that information as they travel from page to page within the ap
Implementing a "Keep Me Logged In" feature means you need to define exactly what that will mean to the user. In the simplest case, I would use that to mean the session has a much longer timeout: 2 days (say) instead of 2 hours. To do that, you will need your own session storage, probably in a database, so you can set custom expiry times for the session data. Then you need to make sure you set a cookie that will stick around for a few days (or longer), rather than expire when they close the browser.
I can hear you asking "why 2 days? why not 2 weeks?". This is because using a session in PHP will automatically push the expiry back. This is because a session's expiry in PHP is actually an idle timeout.
Now, having said that, I'd probably implement a harder timeout value that I store in the session itself, and out at 2 weeks or so, and add code to see that and to forcibly invalidate the session. Or at least to log them out. This will mean that the user will be asked to login periodically. Yahoo! does this.