We use Microsoft HTML Help Workshop to generate the Windows HTML Help format of the manual, and of course PHP itself to do some additional conversions and formatting.
Using XML as a source format gives us the ability to generate many output formats from the source files, while only maintaining one source document for all formats. The tools used for formatting HTML and TeX versions are Jade, written by James Clark and The Modular DocBook Stylesheets written by Norman Walsh. We use Microsoft HTML Help Workshop to generate the Windows HTML Help format of the manual, and of course PHP itself to do some additional conversions and formatting.