The main part of what I get so far is that the documentation is misleading. There are the calendars registered in PHP and those within CalDAV/Sabre. The official Calendar app will only show the CalDAV ones. The CalDAV remote.php backend will most probably not show the PHP-based calendars as well.
The question is not, what your intention is related to the calendars, @susnux. There is at least a workaround. I fear this a combination of a bug and a feature request in fact:
The DAV/calendar backend seems not to work with the PHP-registered calendars (rendering the documentation misleading).
Once a calendar is registered I had problems to see it with the documented PHP methods (see my own calender registered). I will most probably build a test case repository. From there I might be able to file a bug report/feature request.
Sorry, that I cannot give you a better report. At least there is a workaround possible to be used.
I wsa busy these days like hell due to my day-to-day obligations. I will try to fix some of the open issues on my personal agenda soon. One part is to have a look at the calendar thing. So, long story short, I have not done much since the PR.