This book is for professional Web developers who want to make WordPress work exactly how they and their clients want. WordPress has already proven an exceptional platform for building any type of site from simple static pages to networks of full-featured communities. Learning how to code plugins will help you get the most out of WordPress and have a cost-effective approach to developing per-client features.

This book is also for the code freelancers who want to broaden their skill portfolio, understand the inner workings of WordPress functionality, and take on WordPress gigs. Since WordPress is the most popular software to code and power websites, it is crucial that you understand how things run under the hood and how you can make the engine work your way. Learning how to code plugins will be a priceless asset to add to your resume and business card.

Finally, this book is for hobbyist PHP programmers who want to tinker with how their WordPress blog works, discover the infinite potential of lean and flexible source code, and how they can interact with the flow of events. The beauty of open source is that it's easy to learn from and easy to give back in turn. This book will help you take your first step into a community that will welcome your creativity and contribution.

Simply put, this book is for anyone who wants to extend the way WordPress works, whether it is for fun or profit.

This book assumes you already have a Web server and WordPress running. For your convenience it is preferred that your Web server runs on your localhost, as it will be easier to modify plugin files as you read through the book, but an online server is also fine.

Code snippets written in PHP are the backbone of this book: You should be comfortable with reading and writing basic PHP code or referring to PHP's documentation to fill any gaps in knowledge about fundamental functions. Advanced PHP code tricks are explained, so you don't need to be a PHP expert.

You will need to have rudimentary HTML knowledge to fully understand all the code. A basic acquaintance with database and MySQL syntax will help with grasping advanced subjects. To make the most of the chapter dedicated to JavaScript and AJAX, comprehension of JavaScript code and jQuery syntax will be a plus.

As of this writing, WordPress 3.1 is around the corner and this book has been developed alongside this version. Following the best coding practices outlined in this book and using built-in APIs are keys to future-proof code that will not be deprecated when a newer version of WordPress is released. We believe that every code snippet in this book will still be accurate and up-to-date for several years, just as several plugins we coded many years ago are still completely functional today.

This book is, to date, one of the most powerful and comprehensive resources you can find about WordPress plugins. Advanced areas of the many WordPress APIs are covered, such as the Rewrite APIs, cron jobs, and Custom Post Types. This book is divided into three major parts. Reading the first three chapters (Introduction, Plugin Foundations, and Hooks) is required if you are taking your first steps in the wonders of WordPress plugins. Chapters 4 through 7 will cover most common topics in coding plugins, and understanding them will be useful when reading subsequent chapters. The remaining chapters cover advanced APIs and functions, can be read in any order, and will sometimes refer to other chapters for details on a particular function.

To help you get the most from the text and keep track of what's happening, we've used a number of conventions throughout the book.

As for styles in the text:

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