The DOMXML extension has this kind of documentation.
No, DOMXML only supports OO-Syntax, not OO and procedural style.
Also class->method is hard to read instead to have a list of properties and methods for different objects.
Well, the dir class has some true OO documentation, but there is not much real OO docs in the XML sources.
The dir class uses classsynopsys: http://www.docbook.org/tdg/en/html/classsynopsis.html
This enables you to provide an overwiev of the class, but does not let you add documentation for methods or attributes. I would suggest you document the mysqli class with this syntax on the intro page of the mysqli section, and then note on each function page that there is an OO syntax to use it.
Unfortunately I have never used DocBook to document OO stuff, so I have no personal experience.
Goba
