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:

<config>
    <frontend>
        <routers>
            <checkout>
                <args>
                    <modules>
                         <My_Module before="Mage_Checkout">My_Module_Checkout</My_Module>
                    </modules>
                </args>
            </checkout>
        </routers>
    </frontend>
</config>

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/