if(!isset($_SESSION['user_id'])) { header('Location: login.php'); exit; }In this example, the function checks if the user is logged in by checking if a session variable exists for the user id. If the user is not logged in, they are redirected to the login page. Package library: This function is a core PHP function, meaning that it is included in the PHP installation as a built-in function. Therefore, it does not require any package library to be installed.