Example
Helpers should extend from Mage_Core_Helper_Abstract:
# File: app/code/local/Vendor/Package/Helper/Data.php
class Vendor_Package_Helper_Data extends Mage_Core_Helper_Abstract
{
public function multiply($a, $b)
{
return $a * $b;
}
}
To be able to access is via Mage::helper you need to define a helper alias in a config.xml file to allow the Magento autoloader to find your class:
<!-- File: app/code/local/Vendor/Package/etc/config.xml -->
<global>
<helpers>
<alias_here>
<class>Vendor_Package_Helper</class>
</alias_here>
</helpers>
</global>
Assuming your module is correctly configured and you have cleared your cache, you should now be able to use your helper like so:
$result = Mage::helper('alias_here')->multiply(2, 4); // int(8)
Note: if you're using a Data class, its helper name is implied if you don't specify one. For example, the following two examples are identical:
Mage::helper('alias_here');
Mage::helper('alias_here/data');
