WordPress uses PHP and provides an API with its own functions. The API has grown over time, and now covers all possible methods of communication between WordPress and the plugin.

This book will cover all the aspects of the API with practical examples, and the development of six concrete plugins (chapters 2-7). Through step-by-step guides to creating these plugins, you will discover how to use API in typical situations, from displaying output on the site in the beginning, to turning WordPress into a CMS at the end.

Each new chapter and each new plugin will introduce different features of WordPress, and how to put them to good use, thereby allowing you to gradually advance your knowledge.

As much as WordPress team takes care of the WordPress core, you need to take care of security in your plugins. Always remember that your plugin is given the ultimate authority on the user's site. This is a great feature that allowed WordPress to become popular; but with that it also carries a great deal of responsibility.

This book will show the methods and functions best suited to the given situations, and underline the security implications. There are not too many issues to worry about, so make sure you remember the implications and best practices and apply them to your plugins.

Once the development work is done and the plugin is finished, a lot of authors face the question: What now? If you plan to release the plugin to the public, there are a few extra steps you need to take care of.

This book will give you practical tips in this area—refer to Chapter 8.

It will cover the process that takes you from the end of the development phase to the first users coming back to you with feedback and questions. I have learned much from my own plugin development experience, and I will also be referencing some of the resources available online .

This is the first plugin we will be developing. This plugin will show a Digg button in your blog posts.

The purpose of this plugin will also be to introduce you to the basic concepts of WordPress plugin development.

We will access some of the basic WordPress API functions, and talk about filters and actions which are the WordPress mechanisms for controlling the workflow of the site.

The second plugin comes from the idea of making the default blogroll look a little bit more exciting.

We will expand our knowledge of WordPress API, but also introduce two very important allies in WordPress plugin development, namely jQuery and Ajax.

jQuery and Ajax technologies help our plugins become more engaging, and we will learn the basic concepts of using them through a series of practical examples.

This chapter is all about WP Wall; a plugin that creates a shoutbox on your blog's sidebar, where users can leave comments and shouts.

The chapter teaches important lessons, including how to create a widget and interact with the WordPress database. It also expands the usage of Ajax to include dynamic interaction with the user.

I will also try to engage you to think about WordPress plugin development in a slightly different way.

If plugins can be classified by beauty, this would be the most beautiful plugin in the book. It will display your blog archives in a way you probably have not seen before.

During the creation of the plugin, we will explore how to interact with posts, create plugin options, and manage them through a settings page in the administration. We will then use all that knowledge to produce a beautiful representation of your blog's archives.

Insights plugin will add to the productivity of the blog owner by offering quick access to common information in the Write Post screen.

In this chapter, we will learn how to change the WordPress Write Post screen and interact with the built-in TinyMCE editor. We will use Ajax to get information delivered directly into our post while we write it!

The final plugin of the book will see us working more closely with the WordPress backend, and explore ways to transform WordPress into a versatile Content Management System (CMS).

We will learn how to customize the administration menus and pages, use custom fields to store additional information, explore user capabilities system, and use localization.

After all the hard work in developing those plugins, comes a chapter dedicated to all the post-development work (after-hour party!).

The purpose of this chapter will be to explain localization, documentation, code management, and plugin promotion. It will also give you some information regarding WordPress MU development. Wordpress MU is a multi user version of WordPress that is becoming more popular each day.

Obviously, we will need WordPress installed. It is advisable to have a separate version of WordPress for development purposes (that is, do not use your blog as a testing ground; your visitors will not like it very much).

To prepare your local version, you can always download the latest version of WordPress from http://wordpress.org/download/.

Then you need to decide if you are going to host it on the Internet or locally. I prefer to have it on Internet as I can easily show it to other people if necessary, no matter where I am at that moment. The advantage of local installation is that it works much faster. You can of course combine both.

If you are going to set it up on your local computer, and you are using Windows, using packages such as EasyPHP (http://www.easyphp.org) will make your job much easier. This is a software package that incorporates PHP and MySQL for Windows systems, and is free to use.

Installing WordPress is a simple matter. I recommend going to http://codex.wordpress.org/Installing_WordPress for the latest guide to the installation process.

If you would like to set up the latest development version, I suggest you read the section Using SVN in Chapter 8.

Obviously, you are going to need a text editor. Any editor will do, and you probably have a preference by now.

You do not need your text editor to do anything fancy. Indentation support and syntax highlighting for PHP, HTML and JavaScript is almost all you need from an editor at this point.

I have been using UltraEdit for years, and it has been always my weapon of choice for a quick, small, and powerful editor. There also are other text editors that are available for free, such as, Netbeans IDE for PHP, Zend Studio, and so on.

The second thing you will need to have is an FTP client. Uploading plugins can be a boring job if you do it from the FTP command line; so visual FTP client is advisable.

For this matter, I've been using Total Commander as a proven tool. Again, you might have your own preference.

Finally, since we are developing web based software, we will need a web browser. You will be spending a lot of time in the browser window; so make sure you are comfortable with it.

I do not want to instigate browser wars at this moment, but there is one browser that is very flexible and has tons of add-ons, much like WordPress in that respect, and that is Firefox.

There are some pretty cool add-ons for Firefox that can really increase productivity to the level of rapid web development. This step is optional if you do not use Firefox, but I would nevertheless recommend giving it a try.

Firebug

The most important Firefox add-on we want to install is called Firebug (you can download it from http://getfirebug.com).

Firebug allows you to preview, edit and debug CSS, HTML, JavaScript and Ajax requests in real time. This is really a great time saver. For example, when it comes to debugging, JavaScript or misaligned CSS can be really troublesome, without the use of features provided by Firebug.

In the next picture, you can see an example of Firebug in action, debugging JavaScript with a breakpoint in the execution of the script.

Ubiquity

This is an add-on produced by Mozilla Labs and it can do a lot of fancy stuff. But for our purposes, we are going to use it for two simple things—fast search of PHP and WordPress documentation.

You will need to install Ubiquity from https://wiki.mozilla.org/Labs/Ubiquity and also install these two plugins:

1. PHP Search: http://npattison.com/ubiquity/ubiq_php.html
2. WP Codex Search: http://www.prelovac.com/vladimir/ubiquity-plugins/wp-codex-search

The way these two Ubiquity commands work is that when you press Ctrl and Space in the browser (this opens Ubiquity window) and then type php search_phrase or wp search_phrase, it takes you immediately to the PHP or WordPress documentation for the given phrase.

The amount of time this can save is huge, especially if you need to take a peek at the documentation every minute or so like I do (I can't keep everything in my head!).

Screengrab

You can grab Screengrab from http://www.screengrab.org, and this plugin allows you to take screenshots from your browser window easily. It can capture an entire page, a visible portion, or a user selection on the page.

Once you get used it, you will use it extensively for creating images for your plugin documentation. I have used it for almost all the images in this book.

When you want to install a plugin, you need to follow these simple steps:

If you intend to manually update the plugin files on a live site, it is advisable to deactivate the plugin first using the Plugins menu.

Strictly speaking, on a development blog where you are the only user, this can be skipped to save time.

If you need more information regarding plugin management, you can refer to the latest WordPress manual found at http://codex.wordpress.org/Managing_Plugins.

During the plugin development, you will more or less need to refer to documentation.

Luckily, you will have this book on your hand either in paper or in a PDF form, so you can take a quick look. Then, you also have the WordPress online documentation found at http://codex.wordpress.org/Main_Page.

Due to the rapid speed at which WordPress has been developing, not all the functions have been documented.

So, I keep a copy of the latest version of WordPress locally. This way, I am able to find the reference to the function in the WordPress core quickly, and figure out how it is used.

I also tend to keep a local copy of the entire plugin repository on my computer (you can find more details on how to do that in Chapter 8) so that I can search through the entire plugin repository and find instances of this function used by other plugin authors (and you can always find new and creative uses as well).

Using other plugins for this purpose can really save a lot of time, and it is often the easiest way to learn how to perform certain functionalities.