From its very first versions, WordPress has always been designed as a very open platform. This openness has been exemplified not only through its open source licensing and distribution model, but also its open plugin architecture, providing developers with the ability to deliver an even richer experience to its users.

While a basic WordPress installation provides a great amount of functionality that continues to expand from one release to the next, users often have the need to add one more feature to make it the perfect website management system. This is where the plugins come into play. They can fill this gap by augmenting or manipulating virtually any aspect of a WordPress website's display and administrative tasks.

Just like WordPress, plugins are written in the PHP programming language, which is structurally similar to more traditional languages such as C...

The first step of creating a WordPress plugin is to create a PHP file inside of the plugins directory and add the necessary information to have it recognized by the system. This first recipe shows you how to create a basic plugin file for WordPress and how to see and activate this new extension from the administration interface.

You should have access to a WordPress development environment, either on your local computer or a remote server, where you will be able to load your new plugin files.

1. Navigate to...

A common action performed by plugins is to add extra content to the header of visitor-facing pages generated by WordPress. This recipe shows you how to register an action hook function to be able to add such additional content. To make this example more concrete, we will use the Google Analytics page header JavaScript code that so many people use to get good page view statistics for their site.

1. Navigate to the WordPress plugin directory of your development installation.
2. Create a new directory called ch2-page-header-output.
3. Navigate to this directory and create a new text file called ch2-page-header-output.php.
4. Open the new file in a code editor and...

On occasion, plugins need to refer to external files (for example, images, JavaScript, or jQuery script files) that are stored in the plugin directory. Since users are free to rename a plugin's folder or even install plugin files straight into the WordPress plugin directory, paths to any external files must be built dynamically based on the actual plugin location. Thankfully, a number of utility functions are present to simplify this task. In this recipe, we will write a simple plugin that will add a favicon meta tag to a website's header, pointing to an image file located in the plugin directory.

1. Navigate to the WordPress plugin...

Beyond adding functionality or content to a site, the other major task commonly performed by plugins is to augment, modify, or reduce information before it is displayed on the screen. This is done by using WordPress filter hooks, which allow plugins to register a custom function through the WordPress API to be called, since content is prepared before it is sent to the browser. In this recipe, you will learn how to implement your first filter callback function to modify the contents of the generator meta tag that is output as part of the site header.

1. Navigate to the WordPress plugin directory of your development installation.
2. Create a new directory...

After making a number of changes to the page header, the generator meta tag, and the site favicon, this recipe takes a more active role by adding a link to each post or page, allowing visitors to email a link to the item that they are currently viewing. This functionality is implemented using a filter hook attached to the page and post content, allowing our custom function to append custom output code to all entries that get displayed on the screen.

1. Navigate to the WordPress plugin directory of your development installation.
2. Create a new directory called ch2-email-page-link.
3. Navigate to this directory and create a new text file called ch2-email...

After creating two filter functions that append text to the existing content, this recipe shows you how to modify the page content before it is displayed on the screen. More specifically, the following plugin will expand on the Google Analytics header plugin created earlier and add a JavaScript function to all the links that are included in posts and pages to track when they are clicked by visitors.

You should have already followed the Adding output content to page headers using plugin actions recipe to have a starting point for this recipe and the resulting plugin should be active in your development site. Alternatively, you can download...

As you transcribe code segments from the pages of this book or start writing your own plugins, there is a strong chance that you will have to troubleshoot problems with your code or have trouble working with data that your plugin is meant to manipulate. This recipe shows the basic techniques to identify and quickly resolve these errors while creating a plugin that will hide an item from the navigation menu for users who are not logged in to your site.

1. Navigate to the WordPress plugin directory of your development installation.
2. Create a new directory called ch2-nav-menu-filter.
3. Navigate to this directory and create a new text file called ch2-nav-menu...

Shortcodes are a very popular tool in WordPress that allow users to easily add content generated by plugins or themes to any page or post without needing to be familiar with PHP code and editing theme template files. As they are very simple to create, shortcodes can also be used to easily automate the output of content that repeatedly needs to be included in your site's content.

This recipe explains how to create a new custom shortcode that will be used to quickly add a link to a Twitter page in any post or page, automating a repetitive task.

1. Navigate to the WordPress plugin directory of your development installation.
2. Create a new directory called ch2-twitter-shortcode...

While simple shortcodes already provide a lot of potential to output complex content to a page by entering a few characters in the post editor, shortcodes become even more useful when they are coupled with parameters that will be passed to their associated processing function. Using this technique, it becomes very easy to create a shortcode that accelerates the insertion of external content in WordPress posts or pages by only needing to specify the shortcode and the unique identifier of the source element to be displayed.

We will illustrate this concept in this recipe by creating a shortcode that will be used to quickly add Twitter feeds to posts or pages.

1. Navigate...

A different type of shortcode is available in WordPress that encloses content in posts and pages. Using a syntax similar to HTML tags, enclosing shortcodes can be used to identify parts of an item's content that need to be treated in a special way. For example, it is possible to use this type of shortcode to style a part of the post.

As an example of how to create enclosing shortcodes, this recipe shows you how to create a set of tags that will identify part of a post or page that should only be shown to visitors that are logged in to a site. In this way, the shortcode acts similarly to a filter hook, with the added bonus that you do not need to parse for instances of these tags, as would normally be done in a filter.

When a plugin adds custom content or inserts styling tags to a post or page's existing content, as was done in the previous recipe showing how to create an enclosing shortcode, it usually needs to load a custom style sheet to style these new elements. This recipe shows how to add a style sheet in the WordPress style queue to format the private output created in the previous recipe. This queue is processed when the page header is rendered, listing all the style sheets that need to be loaded to display the site correctly.

You should have already followed the Creating a new enclosing shortcode recipe to have a starting point for this recipe and the resulting plugin...

So far, all plugin examples that have been covered in this chapter have been written using the procedural PHP programming style, with all the functions declared directly in the main body of the plugin and the hook registration functions having direct access to these functions.

WordPress can also be written using an object-oriented PHP approach. This recipe explains how to convert the code from the previous recipe into a class-based version of the same functionality.

You should have already followed the Loading a style sheet to format plugin output recipe to have a starting point for this recipe. Alternatively, you can download the resulting code (Chapter 2/ch2-private...