session_start(); function check_login() { if(!isset($_SESSION['user_id']) || empty($_SESSION['user_id'])) { header("Location: login.php"); exit; } } check_login(); // continue with restricted access codeIn this example, the check_login function starts a session and checks if the user_id session variable exists and is not empty. If the user is not logged in, it redirects them to the login page. This function can be used in various contexts, such as restricting access to certain pages or APIs. The implementation details may differ, but the purpose of the check_login function remains the same. As for the package library to use for this function, PHP comes with built-in session handling functions that can be used for basic authentication. However, for more advanced authentication and security features, libraries like Laravel's Auth or Symfony's Security component could be used.