Paul Jones has posted about a new tool he's worked up specifically for authors looking to write using Markdown and wanting it to generate out like DocBook results. His tool, Bookdown, uses Markdown and JSON files instead of XML configurations.
Yes, I know, there’s a ton of static site generators for PHP out there already [...but they're] not DocBook-like documentation. By “DocBook-like”, I mean (among other things) numbered headers, auto-generated tables-of-contents on their own pages, hierarchical multi-page presentation, and the next/previous/up linking at the top and bottom of pages.
[...] So: Bookdown. This scratches my particular itch, with very few dependencies. Bookdown, although it can be used as a site generator, is only incidentally a site generator. What it really is is a page generator, with the idea that you can integrate the pages into any other site you want.
The library is separate from the project and is written to use a dependency injection methodology to keep things decoupled and well-structured. If this sounds interesting either for personal use or if you'd like to check out the code, head over to the project site for more information.