The PHP session system lets you store securely data in the
$_SESSION global array. A typical example is to store the user’s identifier in the session when they type in their password:
if ($user = try_login($login, $password)) $_SESSION['user'] = $user;
Then, you can access that information on all other pages:
if (isset($_SESSION['user'])) // logged in ! echo user_name($_SESSION['user']);
The data is stored on the server, so there is no risk of tampering (on the other hand, mind your disk usage).
Starting the session lets the current request use
$_SESSION. If this is the user’s first visit, the array will be empty and a new session cookie will be sent for you.
Closing the session merely prevents the current request from using
$_SESSION, but the data stays around for the next requests.
Destroying the session throws away all the data, forever. The sessions are destroyed a certain duration after the last visit (usually around 30 minutes).