session_start(); if(isset($_SESSION['user_id'])) { // User is logged in, show content here } else { // User is not logged in, show login form header('Location: login.php'); exit(); }In this example, we are checking if the 'user_id' session variable exists. If it does, we assume the user is logged in and we can display the content they requested. If it doesn't exist, we redirect them to the login page. The PHP Session logcheck library is typically included in the core PHP package and is enabled by default. However, there are also third-party libraries that provide additional functionality and security features for session management. Some popular packages include Aura.Session and Symfony HttpFoundation.