How to check which session is registered in PHP?

I am creating a site and I'm using sessions to manage the user log in status. When a user logs in, I create a session id for the user like so:

$_SESSION['username'] = "username";

How can I check if a user is logged in, and if so, who it is?

From stackoverflow

• If, after you have authenticated the user you are writing $_SESSION['username'] to the the session superglobal, all you need to do is double check the existance of a session username:

   session_start()
   if (empty($_SESSION['username'])) {
        // user is not logged in
   }
  

  Figuring out who is logged in is as simple as checking what username you have stored in the $_SESSION['username'] variable that we checked earlier. If you wanted to save their real name from a database result, or other information, just dump that information into the $_SESSION['data_type'] array as well.

  The session variables should automatically expire in 24 minutes (PHP default, I think) of inactivity, in which case you should expect the user to login again. If you want to log the user out immediately, check out session_destroy().

  Paul Dixon : The configuration value session.gc_maxlifetime governs session length, the default value, oddly, is 24 minutes (1440 seconds).

  chuckg : Thanks Paul, I updated the answer.