Hello Readers,
Sometimes you need to do some changes in magento core files. It is a good habit of every Magento programmer to override the core file instead of making change in core file.
In this tutorial, i will explain how to override magento blocks, controllers & Models.

Overriding Magento Blocks
Path: "app/code/local/Namespace/Module/etc/config.xml" and paste the following contents in that file.

<config>

 <global>

      <blocks>

           <catalog>

            <rewrite>

             <view>Namespace_Module_Block_View</view>

            </rewrite>

           </catalog>

    </blocks>

 </global>

</config>

Now, create a block file "app/code/local/Namespace/Module/block/View.php"

class Namespace_Module_Block_View extends Mage_Catalog_Block_Category_View

{

public function getProductListHtml()

  {


    return $this>getChildHtml('product_list');

  }

}

Override Magento Controllers:
Path: "app/code/local/Namespace/Module/etc/config.xml"

<config>

 <frontend>
    <routers>
            <checkout>
                <args>
                    <modules>
                        <Namespace_Module before="Mage_Checkout">Namespace_Module</Namespace_Module>
                    </modules>
                </args>
            </checkout>
        </routers>

 </frontend>

</config>

Now, create a controller file "app/code/local/Namespace/Module/controllers/CartController.php" and paste the following contents in that file.

<?php

require_once 'Mage/Checkout/controllers/CartController.php';
class Namespace_Module_CartController extends Mage_Checkout_CartController

{

// some code

}

Overriding Magento Models

(path of config file of your custom module ):
Path: "app/code/local/Namespace/Module/etc/config.xml" and paste the following contents in that file.

<config>
  <global>

 <models>

       <checkout>

        <rewrite>

             <cart>Namespace_Module_Model_Cart</cart>

        </rewrite>

       </checkout>

 </models>

 </global>

</config>

Now, create a Model file "app/code/local/Namespace/Module/Model/Cart.php" and paste the following contents in that file.
 

class Namespace_Module_Model_Cart extends Mage_Checkout_Model_Cart

{

// override code in new class

}