How can I override a Magento controller?

Question

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 t

Answer 1

Anything besides modifying the core is good in my opinion. With that said create a simple module with a controllers directory and etc with config.xml:

    
        
            
                
                    
                         My_Module_Checkout
                    
                
            
        
    

See here for more details on how to extend the frontend controller: http://prattski.com/2010/06/24/magento-overriding-core-files-blocks-models-resources-controllers/