Extending / Overriding Magento controller

We can override any controller using our custom extension, so you will need to create below extension files and folders:-

For ex. here we are overriding checkout cart controller.

• app/etc/modules/Namespace_YourModuleName.xml: It's a file used to enable our custom module.
• app/code/local/Namespace/YourModuleName/etc/config.xml: It's a module configuration file in which we'll set up controller class overriding using certain tags as per the Magento conventions.
• app/code/local/Namespace/YourModuleName/controllers/CartController.php: It's a custom controller class file which we'll use to override the core "Cart" controller class.

In your app/etc/modules/Namespace_YourModuleName.xml file, write below code:

<?xml version="1.0" encoding="UTF-8"?>

<config>

<modules>

<Namespace_YourModuleName>

<active>true</active>

<codePool>local</codePool>

</Namespace_YourModuleName>

</modules>

</config>

In your app/code/local/Namespace/YourModuleName/etc/config.xml file, write below code:

<?xml version="1.0" encoding="UTF-8"?>

<config>

<modules>

<Namespace_YourModuleName>

<version>0.1.0</version>

</Namespace_YourModuleName>

</modules>

<frontend>

<routers>

<checkout>

<args>

<modules>

<Namespace_YourModuleName before="Mage_Checkout">Namespace_YourModuleName</Namespace_YourModuleName>

</modules>

</args>

</checkout>

</routers>

</frontend>

</config>

In your app/code/local/Namespace/YourModuleName/controllers/CartController.php file, write below code:

<?php

require_once 'Mage/Checkout/controllers/CartController.php';

class Namespace_YourModuleName_CartController extends Mage_Checkout_CartController

{

public function addAction()

{

echo 'I successfully Override Cart Controller';

parent::addAction();

}

public function indexAction()

{

echo 'I successfully Override Cart Controller';

parent::addAction();

}

}

?>