When taking your first steps to learning a programming language, it’s always good to have a few reference books on hand to help out when you get stuck. Now some users may find e-Books a sufficient source of information, while others (me included) may find having hard copy books a much better way of referencing and sourcing new information about PHP.

I’ve compiled a list of some good books which helped me learn about object oriented PHP.

PHP in Action: Objects, Design, Agility

If I had to recommend 1 book for learning object oriented PHP, I would suggest this book. It covers a wealth of information regarding PHP in an object oriented context. Chapters ranging from agile methodologies and refactoring, to creating website interfaces and database classes. Definitely a book worth buying in either paperback or eBook!

PHP and MySQL Web Development (4th Edition)

If you are looking for a general PHP, MySQL development book look no further. Ranked number 1 on amazon for PHP books, it’s a must read for a general rounding of the PHP language. It does go into object oriented PHP in later chapters also. This book was one of the first PHP books I ever read cover to cover, and I always come back to it, truly an indispensable resource.

PHP 5 Power Programming

This book is certainly not for the faint hearted. If you already have a pretty comprehensive understanding of PHP and Object Oriented Programming, then take a look at this book. It explores XML/SOAP integration, as well as design patterns. It also includes a 90 day trial key for Zend Studio, if you are still undecided on an IDE.

Professional PHP5 (Programmer to Programmer)

The lead author of this book, Edward Lecky-Thompson says it all in his review of his own book, "If you don't know PHP inside and out, this book is not for you". Professional PHP5 goes where few other PHP books go, it examines enterprise level architecture, design patterns and many other fairly advanced PHP techniques, certainly a book for seasoned professionals. Before reading this book, I recommend reading Beginning PHP5 (Programmer to Programmer), which is the baby brother to this book.

Advanced PHP Programming

A rather generic advanced PHP book, goes through the standard Object Oriented PHP techniques as well as outlining and defining various features of the PHP5 object model. Certainly another good book to have lying around for a reference here and there.

To Finish Up

Above are a few books that have helped me out in the past, if you are a beginner PHP programmer then some of these books may go over you're head, while some of them would provide a good starting point for learning object oriented PHP. If you are a seasoned PHP developer, these books will help solidify your PHP knowledge and understanding.
With PHP, generally there are many different ways of achieving a goal, however not all solutions are as good as others, having these books readily available allows developers to think of PHP solutions from different perspectives and offers new learning areas.

If you have read a good PHP book, or know of other valuable paperback resources. Feel free to post them below.