It could be that session variables aren't working. Try setting a session variable on one page and retrieving it on another and see if it works. If not, then it is likely an issue with the session_save_path in your php.ini file.
Set a session variable with:
$_SESSION["test"] = "Works";
Then on a second page check the value with:
If you don't see"Works" on the second page, then there is an issue with your session_save_path and your hosting provider should fix it for you.