Hi Rolf,
Yes, I am enjoying retirement, immensely.
There were a bunch of plugins I loved to use in my courses. Two that I used a bunch in my courses, were put up for adoption, as the original developers could no longer spend time keeping them updated. I had managed to keep them working for my students so I offered to take over responsibility for them. That's where my "development" work occurs, adding missing capabilities to them.
As for PHP, at the time of adoption, I knew practically nothing about it. I just spent a lot of time digging through the Development part of Moodle documentation, looking at other modules, trial and error attempts, plus I installed and used the Code Checker plugin.
There is a tutorial link that you can look at here.
Have fun,
AL