When you rewrite a helper class, it will only take effect when you use Mage::helper('...').

However, most helper classes extend from Mage_Core_Helper_Data directly and then, the original class is used :(

For example: class My_Custom_Helper_Class extends Mage_Core_Helper_Data { }

If you want to rewrite such a class in all cases, you can use the ugly method to copy it into app/code/local/Mage/Core/Helper/Data.php and do your edits there.

It is not nice style, but if you want to extend a class very high up in the hierarchy, you have no other choice as far as I know.

When you rewrite a helper class, it will only take effect when you use Mage::helper('...').

However, most helper classes extend from Mage_Core_Helper_Data directly and then, the original class is used :(

For example:

class My_Custom_Helper_Class extends Mage_Core_Helper_Data {
}

If you want to rewrite such a class in all cases, you can use the ugly method to copy it into app/code/local/Mage/Core/Helper/Data.php and do your edits there.

It is not nice style, but if you want to extend a class very high up in the hierarchy, you have no other choice as far as I know.