PHP: The Right Way (Phil Sturgeon/Josh Lockhart)
This book aims to introduce new PHP developers to some topics which they may not discover until it is too late, and aims to give seasoned pros some fresh ideas on those topics they 've been doing for years without ever reconsidering.

PHP Programming (Penn Wu)
You will learn PHP programming language by understanding how its primary functions work, how it interacts with HTML, and how its nature of being a server-side scripting language differs from a client-side language, such as JavaScript.

AJAX and PHP: Building Responsive Web Applications (C. Darie)
Assuming a basic knowledge of PHP, XML, JavaScript and MySQL, this book will help you understand how the heart of AJAX beats and how the constituent technologies work together.

PHP Reference: Beginner to Intermediate PHP5 (Mario Lurig)
This book is a collection of over 250 PHP functions with clear explanations in language anyone can understand, followed with as many examples as it takes to understand what the function does and how it works..

PHP Best Practices: A Guide for Common and Confusing PHP Tasks
This book is an attempt to compile a set of basic instructions for what can be considered best practices for common and confusing issues and tasks in PHP.