PHP Game Programming, the bookI came across this book on Amazon just now:
"PHP Game Programming" offers you the introduction you need to begin creating your own online games. You?ll be amazed at the games you can create with this powerful and completely free development tool! Dive right in as you begin with coverage of server configuration and the major features of PHP. Then you're off and running as you use PHP to create and manipulate graphics, develop a chess game using a non-relational database, and send and receive data through sockets. Put your new skills to use as you create your own massively multiplayer online game! From the basics of PHP and HTML to the exciting task of creating dynamic terrain and Flash movies, "PHP Game Programming" will help you turn your online game ideas into reality!
Reviews aren't outstanding, but I believe this may be the only book available that is specific to game development with PHP and it is of interest in that respect.