function checkToken($token) { if ($_SESSION['token'] !== $token) { header('HTTP/1.1 401 Unauthorized'); die('Invalid token'); } }In this example code, the function checks whether the token sent in the HTTP request matches the token stored in the session. If they don't match, the function sends a 401 Unauthorized response and terminates the script with an error message. This function is commonly used to prevent Cross-Site Request Forgery (CSRF) attacks, where an attacker exploits a vulnerability in a web application to execute unauthorized actions on behalf of the user. The package library that may be used to implement this function would depend on the specific framework or CMS being used. For example, Laravel, a popular PHP framework, has built-in CSRF protection that automatically generates and verifies CSRF tokens for every form submission. Other libraries that may be used to implement this function include Symfony, Yii, and CodeIgniter.