Usability: disabling "PHP filter" module leaves redundant "PHP code" input format

There is a usability issue here: see http://drupal.org/node/242112.

When the module is first installed, it creates a "PHP code" input format. However, when the module is disabled, it leaves the input format behind (albeit the input format doesn't do anything since its filter has been removed).

I'm not quite sure what it *should* do but it is a bit counter-intuitive for the input format still to be listed on node Edit pages.

If the admin then deletes the PHP code input format this then gets rid of the PHP input format option on node Edit pages. However, if the module is re-enabled, there is no input format using the filter - it has to be created by hand.

Obviously there is also the configuration of the roles allowed to use the input format, at admin/settings/filters/n.

Again I'm not sure how it *should* all work here - perhaps what we have *does* make most sense, but maybe the odd help message (via drupal_set_message()?) would be useful.