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WordPress Plugins allow easy modification, customization, and enhancement to a WordPress blog. Instead of changing the core programming of WordPress, you can add functionality with WordPress Plugins. Here is a basic definition:
WordPress Plugin: A WordPress Plugin is a program, or a set of one or more functions, written in the PHP scripting language, that adds a specific set of features or services to the WordPress weblog, which can be seamlessly integrated with the weblog using access points and methods provided by the WordPress Plugin Application Program Interface (API).
Wishing that WordPress had some new or modified functionality? The first thing to do is to search various WordPress Plugin repositories and sources to see if someone has already created a WordPress Plugin that suits your needs. If not, this article will guide you through the process of creating your own WordPress Plugins.
This article assumes you are already familiar with the basic functionality of WordPress, and PHP programming.
This section of the article goes through the steps you need to follow, and things to consider when creating a well-structured WordPress Plugin.
The first task in creating a WordPress Plugin is to think about what the Plugin will do, and make a (hopefully unique) name for your Plugin. Check out Plugins and the other repositories it refers to, to verify that your name is unique; you might also do a Google search on your proposed name. Most Plugin developers choose to use names that somewhat describe what the Plugin does; for instance, a weather-related Plugin would probably have the word "weather" in the name. The name can be multiple words.
The next step is to create a PHP file with a name derived from your chosen Plugin name. For instance, if your Plugin will be called "Fabulous Functionality", you might call your PHP file fabfunc.php. Again, try to choose a unique name. People who install your Plugin will be putting this PHP file into the WordPress Plugin directory in their installation, wp-content/plugins/, so no two Plugins they are using can have the same PHP file name.
In the rest of this article, "the Plugin PHP file" refers to the main Plugin PHP file, whether in wp-content/plugins/ or a sub-directory.
If you want to host your Plugin on http://wordpress.org/extend/plugins/, you also need to create a readme.txt file in a standard format, and include it with your Plugin. See http://wordpress.org/extend/plugins/about/readme.txt for a description of the format.
It is also very useful to create a web page to act as the home page for your WordPress Plugin. This page should describe how to install the Plugin, what it does, what versions of WordPress it is compatible with, what has changed from version to version of your Plugin, and how to use the Plugin.
Now it's time to put some information into your main Plugin PHP file.
The top of your Plugin's main PHP file must contain a standard Plugin information header. This header lets WordPress recognize that your Plugin exists, add it to the Plugin management screen so it can be activated, load it, and run its functions; without the header, your Plugin will never be activated and will never run. Here is the header format:
<?php /* Plugin Name: Name Of The Plugin Plugin URI: http://URI_Of_Page_Describing_Plugin_and_Updates Description: A brief description of the Plugin. Version: The Plugin's Version Number, e.g.: 1.0 Author: Name Of The Plugin Author Author URI: http://URI_Of_The_Plugin_Author */ ?>
The minimum information WordPress needs to recognize your Plugin is the Plugin Name line. The rest of the information (if present) will be used to create the table of Plugins on the Plugin management screen. The order of the lines is not important.
It is customary to follow the standard header with information about licensing for the Plugin. Most Plugins use the GPL license used by WordPress or a license compatible with the GPL. To indicate a GPL license, include the following lines in your Plugin:
<?php /* Copyright YEAR PLUGIN_AUTHOR_NAME (email : PLUGIN AUTHOR EMAIL) This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA */ ?>
Now, it's time to make your Plugin actually do something. This section contains some general ideas about Plugin development, and describes how to accomplish several tasks your Plugin will need to do.
Many WordPress Plugins accomplish their goals by connecting to one or more WordPress Plugin "hooks". The way Plugin hooks work is that at various times while WordPress is running, WordPress checks to see if any Plugins have registered functions to run at that time, and if so, the functions are run. These functions modify the default behavior of WordPress.
For instance, before WordPress adds the title of a post to browser output, it first checks to see if any Plugin has registered a function for the "filter" hook called "the_title". If so, the title text is passed in turn through each registered function, and the final result is what is printed. So, if your Plugin needs to add some information to the printed title, it can register a "the_title" filter function.
Another example is the "action" hook called "wp_footer". Just before the end of the HTML page WordPress is generating, it checks to see whether any Plugins have registered functions for the "wp_footer" action hook, and runs them in turn.
You can learn more about how to register functions for both filter and action hooks, and what Plugin hooks are available in WordPress, in the Plugin API. If you find a spot in the WordPress code where you'd like to have an action or filter, but WordPress doesn't have one, you can also suggest new hooks (suggestions will generally be taken); see Reporting Bugs to find out how.
Another way for a WordPress Plugin to add functionality to WordPress is by creating custom Template Tags. Someone who wants to use your Plugin can add these "tags" to their theme, in the sidebar, post content section, or wherever it is appropriate. For instance, a Plugin that adds geographical tags to posts might define a template tag function called geotag_list_states() for the sidebar, which lists all the states posts are tagged with, with links to the state-based archive pages the Plugin enables.
To define a custom template tag, simply write a PHP function and document it for Plugin users on your Plugin's home page and/or in the Plugin's main PHP file. It's a good idea when documenting the function to give an example of exactly what needs to be added to the theme file to use the function, including the <?php and ?>.
Most WordPress Plugins will need to get some input from the site owner or blog users and save it between sessions, for use in its filter functions, action functions, and template functions. This information has to be saved in the WordPress database, in order to be persistent between sessions. There are two basic methods for saving Plugin data in the database:
See Creating Options Pages for info on how to create a page that will automatically save your options for you.
WordPress has a mechanism for saving, updating, and retrieving individual, named pieces of data ("options") in the WordPress database. Option values can be strings, arrays, or PHP objects (they will be "serialized", or converted to a string, before storage, and unserialized when retrieved). Option names are strings, and they must be unique, so that they do not conflict with either WordPress or other Plugins.
Here are the main functions your Plugin can use to access WordPress options.
add_option($name, $value, $deprecated, $autoload);
Assuming that your Plugin has some options stored in the WordPress database (see section above), you will probably want it to have an administration panel that will enable your Plugin users to view and edit option values. The methods for doing this are described in Adding Administration Menus.
Once you have the programming for your Plugin done, another consideration (assuming you are planning on distributing your Plugin) is internationalization. Internationalization is the process of setting up software so that it can be localized; localization is the process of translating text displayed by the software into different languages. WordPress is used all around the world, so it has internationalization and localization built into its structure, including localization of Plugins.
It is highly recommended that you internationalize your Plugin, so that users from different countries can localize it. There is a comprehensive reference on internationalization, including a section describing how to internationalize your plugin, at I18n for WordPress Developers.
This last section contains some random suggestions regarding Plugin development.