PhD is PHP's very own DocBook 5 rendering system. It is used to convert the PHP Manual and PEAR Documentation into different output formats like XHTML, PDF, Man pages and CHM.
The goal of PhD is to become a fast, general DocBook rendering system. At the moment of writing, PhD is already very fast: It can create the chunked version of PEAR's manual (some 3000 files) in less than a minute on a 2GHz system. It also renders the PHP and PEAR manual flawlessly. It does not support every DocBook 5 tag yet, and using it to render own DocBook files may need some tweaks.