Variable form validation with separate validation config file

[eluser]rebellion[/eluser]
I have a webapp where I'm using a lot of forms for adding and editing data.

To have a nice overview over all the validation that is going on, I've added all the validation rules in form_validation.php in the application config folder.

But I have some questions regarding variable validation. Based on what the user selects in the form, some input fields are required in one combination, but not in others.

Earlier I had all the validation rules in the controller, and did a check on the post values for which fields should be validated for what, but how would I solve that when I have all the rules in a separate config file?