At one time I had PHP working with JBoss AS 5.0.0. The interesting part is usually making certan that the run scripts set the proper java.library.path, and making sure that the path matches the directory structure for where the files are located.
However, ever since I ran across Quercus, I have abandoned using native PHP within JBoss AS because Quercus provides everythign I need for running PHP within JBoss AS and has many benefit above using native PHP.
http://quercus.caucho.com/