How to update a $_SESSION varriable in the after a calculation in the?

For my webshop I want to display the number of items in my shoppingcart, which is located in my : see shopping cart with 37 items

The problem is that my calculation of the number of items happens in the . So I always need to reload my page in order to get the correct number of items to be displayed in my shopping cart. I already tried to use header('Webshop.php'); after my calculation but I get the error: Cannot modify header information - headers already sent by...

Anyone has sugestions?

Kind regards, PJ