If you’re interested in writing DocBook style documentation this tutorial is for you. DocBook is basically SGML (XML) markup that can be output in any of the following formats:
- HTML
- Man pages
- PostScript
- TeX and DVI
- RTF
- TexInfo
DocBook is generally considered the standard for programming documentation (not API docs) but is used in many other disciplines as well. If you’ve ever read PEAR documentation of anything by Eric Raymond, you’ve read DocBook 
December 21st, 2002 at 6:32 pm
Well, the PHP Documentation itself is also written in DocBook
February 13th, 2003 at 3:56 am
Alan Knowles has an excellent summary on how to write documentation for PearDoc at http://news.php.net/article.php?group=php.pear.dev&article=13061