After looking at what Hannes proposed, I think it is simple and fairly usable. I see potential parsing issues in the synopsis section, though. The code portion of that section should be marked somehow, perhaps with:
``php-synopsis int function strpos( string $haystack, mixed $needle, int $offset = 0 ) `` However, I agree that writing custom markdown is quite pointless when we could use rst. I'm not sold on that format either; I'm just saying that if we are going to switch from docbook we need something stable. Rolling our own sounds like a bad idea. It would also be a big transition from docbook to another format. We'd definitely need a translation tool to help us. I'm not sure what is out there, but I can definitely say what is not out there: a docbook to our-custom-markdown formatter. That's another thing to keep in mind.