Overide Magento Class

In this example we build a small custom module that changes a default page behavior. We will change the <title> of all Magento site pages.

<?xml version="1.0"?>
<!-- Filename:  appetcmodulesMsc_Module3.xml -->
<config>
    <modules>
        <Msc_Module3>
            <active>true</active>
            <codePool>local</codePool>
        </Msc_Module3>
    </modules>
</config>

<?php
class Msc_Module3_Helper_Data extends Mage_Core_Helper_Abstract{

}

<?xml version="1.0"?>
<config>
    <modules>
        <Msc_Module3>
            <version>0.0.1</version>
        </Msc_Module3>
    </modules>
    <global>
        <blocks>
            <module3>
                <class>Msc_Module3_Block</class>
            </module3>
        </blocks>
        <helpers>
            <module3>
                <class>Msc_Module3_Helper</class>
            </module3>
        </helpers>
    </global>
    <frontend>
        <layout>
            <updates>
                <module3>
                    <file>module3.xml</file>
                </module3>
            </updates>
        </layout>
    </frontend>
</config>

<?xml version="1.0"?><layout>
    <default>
        <reference name="head">
            <action method="setTitle"><string>Overide default title!</string></action>
        </reference>
    </default>
</layout>