I'm currently going through the Magento Developer Certification Preparation Study Group Guide and doing the exercises. I've come across the following question and I have no idea what to do with it:
Create a dynamic rewrite of the payment/data helper only if the version of Magento is older than version 1.4, and if the ccsave payment option is enabled for the current store.
I can get the version number with Mage::app()->getVersion() and and check if ccsave is is enabled with Mage::app()->getStoreConfig('payment/ccsave/active/') but as far as I can tell one can't use these in their config.xml.
I also thought I might be able to do these checks in the _construct() method but there doesn't seem to be one for Mage_Payment_Helper_Data