Magento 1 : how to override/rewrite a core controller class?

There are multiple approaches but I'll start with how it's not done to clarify some common misconceptions:

1. It's not possible to override controller classes by copying them to app/code/local. This is because controller classes are not loaded by Varien_Autoload, instead the files are explicitly included.
2. It's not recommended to use <rewrite><controller><to> syntax anymore. This is an old technique which is obsolete since Magento 1.3 (see: Overwriting controller vs overwriting action controller request)

Add/override controller actions

To add controller actions to an existing controller, use the following in your config.xml:

<frontend>             <--- area (adminhtml or frontend)
    <routers>
        <checkout>     <--- front name (in admin always "adminhtml")
            <args>
                <modules>
                    <stack_checkout before="Mage_Checkout">Stack_Checkout</stack_checkout>
                                                  ^                ^
                                                  |                |
                                           module to override      |
                </modules>                 (in admin always        |
            </args>                        "Mage_Adminhtml")   your module
        </checkout>
    </routers>
</frontend>

Then create a controller in your module such as

class Stack_Checkout_OnepageController extends Mage_Core_Controller_Front_Action
{
    public function indexAction()
    {
        // here you override checkout/onepage/index
    }
    public function helloAction()
    {
        // here you create a new action checkout/onepage/hello
    }
}

You don't need to extend the original controller class because Magento will look in both classes, in the order defined by before="..."

If you need to extend the original class because you want to reuse other methods from it, you have to include it (remember, controllers are not autoloaded):

require_once(Mage::getModuleDir('controllers','Mage_Checkout') . DS . 'OnepageController.php');

Use observers to modify controller actions

If you don't add new actions, an alternative is to use observers to modify the behavior of existing actions. Every controller action triggers a dynamic "predispatch" event in the form controller_action_predispatch_$FRONTNAME_$CONTROLLER_$ACTION, for example controller_action_predispatch_checkout_onepage_index

In the observer you have access to the controller class itself using

$controller = $observer->getControllerAction();

If you don't want the original method to be triggered, tell Magento to not further dispatch the action:

$controller->setFlag('', Mage_Core_Controller_Front_Action::FLAG_NO_DISPATCH, true);

For the sake of completeness: You can also prevent "postdispatch" events in a similar way, but this is usually not necessary (here is an example where it's useful: XML has extra content):

$controller->setFlag('', Mage_Core_Controller_Front_Action::FLAG_NO_POST_DISPATCH);

Speaking of which, you can also add an observer for controller_action_postdispatch_$FRONTNAME_$CONTROLLER_$ACTION if you want to perform additional actions or modifications of the response after the original action has been executed.

Rewrite:

When we rewrite magento Core classes in our class then we are doing rewrite.

1)Example Of Rewrite : In rewrite you have to create your own class and need to extend core class and you can rewrite magento core functions or your own function and logic there

In your config.xml File

<config>
<frontend>
    <routers>
        <tag>
            <args>
                <modules>
                    <inchoo_tag before="Mage_Tag">Inchoo_Tag</inchoo_tag>
                </modules>
            </args>
        </tag>
    </routers>
</frontend>

In your controller file

require_once(Mage::getModuleDir('controllers','Mage_Tag').DS.'TagController.php');


class Inchoo_Tag_TagController extends Mage_Tag_TagController
{
// some code
}

At Adminhtml

in yourconfig.xml should be

<config>
<admin>
    <routers>
        <adminhtml>
            <args>
                <modules>
                    <inchoo_tag before="Mage_Adminhtml">Inchoo_Tag_Adminhtml</inchoo_tag>
                </modules>
            </args>
        </adminhtml>
    </routers>
</admin>

and your controller class should be

require_once(Mage::getModuleDir('controllers','Mage_Adminhtml').DS.'TagController.php');


class Inchoo_Tag_Adminhtml_TagController extends Mage_Adminhtml_TagController
{
// some code
}

Thank you for Inchoo team For this nice article

http://inchoo.net/magento/overriding-magento-blocks-models-helpers-and-controllers/