First of all, it specifically says 'you may optionally specify a PHP Callback', so I should not have to. Secondly, I think this should be entirely optional. Right now, it seems like something has to be entered there, while I can't imagine that's how it's supposed to be. Maybe make a checkbox or something, so that if you want to use a custom Callback, you can enter it after checking the checkbox?
Could this get fixed?