aren't cookies global?

The redirect function is as it should be and url_helper.php is not corrupt - at least the redirect function (as shown) is not.

"refresh" vs "location" makes no difference on what ultimately happens. redirect() tells to browser to "go to this URL" and the call to exit tells the server to stop executing the current script. Once the script stops then the server forgets all about what it just did.

When the browser is asked for a URL (in this case due to the redirect) it sends any cookies related to that URL. When a valid session cookie is seen during the session class load-up any related session data will be recovered.

It is not clear how/when you are loading the session class. But since the session data is not recovered then sessions are not configured correctly. The config problem could be either the session and/or cookie $config items.

Share those config items and maybe someone will spot a problem.