User session helper class
This is not a SessionHelper but a UserSessionHelper in the first place. A session is a container that could contain anything, not only user credentials. A shopping cart items for example. Therefore you have to either extend its functionality or at least rename a class. 

In 2018 you are obligatory supposed to utilize a PSR-4: Autoloader so not to include your class definitions manually.

What is most important, I don't see any getSession() method. Are you checking $_SESSION array directly? If so, that makes you class essentially incomplete. When writing OOP, you must encapsulate the full functionality, leaving something outside just makes no sense.

Given there is no constructor, consider using static methods, it will save you the very absent constructor call.