The api to interact with enrol/index.php is pretty limited as it just returns an HTML string, this limits what we can do in enrol/index.php when muliple options are available. We had problems on MDL-50332 and MDL-47761 and the current behaviour is not specially usable. For example, when there are multiple enrol_self instances, some of them might not be available, in this enrol_self specific case, can_self_enrol returns true or an error string (which is neither optimal though) we should be able to display (enrol/index.php) the available enrolment methods and the not available ones separately, for this we need to change what the hook returns.

Without breaking backwards compatibility, we should look at deprecating enrol_page_hook while allowing a new API to be used. We can update enrol/index.php as part of this issue too.