I need to check the validity of a coupon code on the checkout/cart page with server-side code.
Magento already ships with a similar check in place. However, I need to add one to see if a user is connected or not: what would be the best way to extend/override that action in Magento?
I know I can copy the controller PHP file to the /app/code/local/
folder tree, but I'm wondering whether there's a better way to do it.