Couple of recent articles looking at how to build content for wireless devices (with examples using PHP): Developing Wireless Content using XHTML Mobile and Introduction to WAP using WML, ASP and PHP.
Some random notes;
Opera does a pretty good job of displaying WML (enough so you’ve got a good idea of what it will look like on a mobile). Also there’s a Mozilla WML Browser plugin (currently 0.6 but getting there).
HAWHAW is a very mature library for building WAP/WML apps with PHP. Gives you a DOM like interface for building pages and handles browser detection.
A starting point for Moblogging
Wireless and Mobile @ phpclasses