Samisa Abeysinghe who is the director of engineering at WSO2 and one of the key leaders of the WSF/PHP project has published a book titled RESTful PHP Web Services.
RESTful PHP Web Services - Samisa Abeysinghe
In Samisa’s Blog He describes the structure and the content of the book in his own words.
If you are developing RESTful web services in PHP, you will find this book will be a great reference.