How can you check if a PHP session exists?

Question

Just wondering how to check if a PHP session exists... My understanding is that no matter what, if I am using sessions, I have to start my files with session_start() to even acc

Answer 1

Store the session_id in $_SESSION and check against it.

First time

session_start();
$_SESSION['id'] = session_id();

Starts a session and stores the randomly given session id.

Next time

session_start();
$valid_session = isset($_SESSION['id']) ? $_SESSION['id'] === session_id() : FALSE;
if (!$valid_session) {
   header('Location: login.php');
   exit();
}

Starts a session, checks if the current session id and the stored session id are identical (with the ternary ? as replacement for the non-existing short circuit AND in php). If not, asks for login again.