WordPress does not use sessions to hold any data being that it is a stateless application. This makes it quite a problem for tasks like a shopping cart, which requires data(the user’s selected product) to be remembered from one page to the next. This means that if you want to use PHP sessions in your plugins or custom modifications of WordPress you will need to do some custom coding.
Luckily the fix is a simple one that anyone can handle! You only need to do a little hacking to enable sessions within WordPress.