Let’s try overriding Mage_Checkout_Helper_Cart helper class in this article.

Step 1 – Create a module in your local folder app/local/Magemonkeys/Checkout

Step 2 – Register your module in etc/modules directory by creating Magemonkeys_Checkout.xml file with the following content:

<?xml version="1.0"?>
<config>
    <modules>
        <Magemonkeys_Checkout>
            <active>true</active>
            <codePool>local</codePool>           
        </Magemonkeys_Checkout>
    </modules>
</config>

Step 3 – Create a config.xml file inside your etc directory of your module directory app/local/Magemonkeys/Checkout/etc/config.xml with the following code:

<?xml version="1.0"?>
    <config>
        <modules>
            <Magemonkeys_Checkout>
                <version>0.0.1</version>
            </Magemonkeys_Checkout>
        </modules>
 
        <global>
           <helpers>
              <checkout>
                  <rewrite>
                      <data>Magemonkeys_Checkout_Helper_Data</data>
                 </rewrite>
              </checkout>
           </helpers>
        </global>
  </config>

Step 4 – Create Data.php file in helper directory of your module directory app/local/Magemonkeys/Checkout/helper/Data.php with the following code snippet:

<?php
class Magemonkeys_Checkout_Helper_Data extends Mage_Checkout_Helper_Data
{
   public function sendPaymentFailedEmail($checkout, $message, $checkoutType = 'onepage')
   {
       return false;
   }
   public function newCheckoutHelperFunction()
   {
        return 'Overridden checkout helper function'
   }
}
?>

Step 5 – You can call your new helper function in phtml file by using the following code:

<?php
     echo $this->helper('checkout')->newCheckoutHelperFunction();
?>