For PHP enthusiasts who are looking to stretch their capabilities, Manning Press has come out with an interesting book called "PHP In Action". Unlike most PHP books, space is not devoted to the basics of web application development, but instead devoted to detailing contemporary development practices and how they can be applied to PHP.
"Agile development", for example, is a catchphrase frequently mentioned in software development circles, but what exactly is it and how can it be applied in PHP? "PHP in Action" goes into detail about agile practices such as proactive refactoring and test-driven development and, when needed, introduces specific tools (such as SimpleTest and PHPUnit). Other interesting topics include the use of design patterns, advanced use of objects, model-view-controller architecture, and object-relational mapping.
If you're happy with your PHP development skills, but haven't been exploring the evolution of development in recent years, you might want to check out this book.
