Purpose: Automatically logs out a user after a period of inactivity.

function checkSessionTimeout()
{
    global $pdo;

    // 10 seconds for testing
    // 1800 seconds = 30 minutes
    $timeout = 1800;

    if (isset($_SESSION['last_activity'])) {

        $inactive = time() - $_SESSION['last_activity'];

        if ($inactive >= $timeout) {

            endUserSession($pdo);

            session_unset();
            session_destroy();

            header(
                'Location: ' .
                BASE_URL .
                '/index.php?timeout=1'
            );

            exit;
        }
    }

    $_SESSION['last_activity'] = time();
}

1. global $pdo; – Brings the PDO connection into scope (it's assumed to be defined outside this function).
2. Sets $timeout to 1800 seconds (30 minutes). The comment mentions 10 seconds is used for testing.
3. If last_activity exists in the session:
Computes $inactive = time() - $_SESSION['last_activity'] If inactive ≥ timeout: Calls endUserSession($pdo) to close the DB record. Clears the PHP session with session_unset() and session_destroy() Redirects to the login page with a ?timeout=1 flag. exit stops further script execution.
4. Always updates $_SESSION['last_activity'] to the current time so activity keeps the session alive.