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{{Languages|
{{en|Writing a Plugin}}
{{ja|Writing a Plugin}}
{{ru|Написание плагина}}
{{zh-hans|开发一个插件}}
{{zh-tw|開發一個外掛}}
}}

= Introduction =

[[Plugins|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 API|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. 

== Resources ==

* To understand how WordPress Plugins work and how to install them on your WordPress blog, see [[Plugins]].
* There is a comprehensive list of articles and resources for Plugin developers, including external articles on writing WordPress Plugins, and articles on special topics, in [[Plugin Resources]].
* To learn the basics about how WordPress Plugins are written, view the source code for well-written Plugins, such as [[Plugins#Default Plugins|Hello Dolly]] distributed with WordPress.
* Once you have written your WordPress Plugin, read [[Plugin Submission and Promotion]] to learn how to distribute it and share it with others.

= Creating a Plugin =

This section of the article goes through the steps you need to follow, and things to consider when creating a well-structured WordPress Plugin.

== Names, Files, and Locations ==

=== Plugin Name ===

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.

=== Plugin Files ===

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 ttfabfunc.php/tt. 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, ttwp-content/plugins//tt, so no two Plugins they are using can have the same PHP file name. 

Another option is to split your Plugin into multiple files. Your WordPress Plugin must have at least one PHP file; it could also contain JavaScript files, CSS files, image files, language files, etc. If there are multiple files, pick a unique name for a file directory and for the main PHP file, such as ttfabfunc/tt and ttfabfunc.php/tt in this example, put all your Plugin's files into that directory, and tell your Plugin users to install the whole directory under ttwp-content/plugins//tt. 

In the rest of this article, the Plugin PHP file refers to the main Plugin PHP file, whether in ttwp-content/plugins//tt or a sub-directory.

=== Readme File ===

If you want to host your Plugin on http://wordpress.org/extend/plugins/, you also need to create a ttreadme.txt/tt 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.

=== Home Page ===

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.

== File Headers ==

Now it's time to put some information into your main Plugin PHP file.

=== Standard Plugin Information ===

The top of your Plugin's main PHP file must contain a standard Plugin information [[File Header|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:

pre
?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
License: A Slug license name e.g. GPL2
*/
?
/pre

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.

The License slug should be a short common identifier for the license the plugin is under and is meant to be a simple way of being explicit about the license of the code.

=== License ===

It is customary to follow the standard header with information about licensing for the Plugin. Most Plugins use the [http://www.gnu.org/licenses/old-licenses/gpl-2.0.html GPL2] license used by WordPress or a license [http://www.fsf.org/licensing/licenses/index_html#GPLCompatibleLicenses compatible with the GPL2]. To indicate a GPL2 license, include the following lines in your Plugin:

pre
?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, version 2, as 
    published by the Free Software Foundation.

    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
*/
?
/pre

== Programming Your Plugin ==

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.

=== WordPress Plugin Hooks ===

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.

=== Template Tags ===

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 ttgeotag_list_states()/tt 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 tt?php/tt and tt?/tt.

===Saving Plugin Data to the Database===

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:

# Use the WordPress option mechanism (described below). This method is appropriate for storing relatively small amounts of relatively static, named pieces of data -- the type of data you'd expect the site owner to enter when first setting up the Plugin, and rarely change thereafter.
# Post Meta (a.k.a. Custom Fields).  Appropriate for data associated with individual posts, pages, or attachments.  See [[Function_Reference/post_meta Function Examples|post_meta Function Examples]], [[Function Reference/add_post_meta|add_post_meta()]], and related functions.
# Create a new, custom database table. This method is appropriate for data associated with individual posts, pages, attachments, or comments -- the type of data that will grow as time goes on, and that doesn't have individual names. See [[Creating Tables with Plugins]] for information on how to do this.

=== WordPress Options Mechanism ===

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.

pre
add_option($name, $value, $deprecated, $autoload);
/pre
: Creates a new option; does nothing if option already exists.
;$name: Required (string). Name of the option to be added.
;$value: Optional (mixed), defaults to empty string. The option value to be stored.
;$deprecated: Optional (string), no longer used by WordPress, You may pass an empty string or null to this argument if you wish to use the following $autoload parameter.
;$autoload: Optional, defaults to 'yes' (enum: 'yes' or 'no').  If set to 'yes' the setting is automatically retrieved by the ttget_alloptions/tt function.

pre
get_option($option);
/pre
: Retrieves an option value from the database.
;$option: Required (string).  Name of the option whose value you want returned. You can find a list of the default options that are installed with WordPress at the [[Option Reference]].

pre
update_option($option_name, $newvalue);
/pre
: Updates or creates an option value in the database (note that ttadd_option/tt does not have to be called if you do not want to use the tt$deprecated/tt or tt$autoload/tt parameters).
;$option_name: Required (string). Name of the option to update.
;$newvalue: Required. (string|array|object) The new value for the option.

=== Administration Panels ===

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]].

== Internationalizing Your Plugin ==

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. 

Please note that language files for Plugins '''IS NOT''' automatically loaded. Add this to the Plugin code to make sure the language file(s) are loaded:

'''load_plugin_textdomain('your-unique-name','/wp-content/plugins/plugin-name/location-of-mo-po-files/');'''

To fetch a string simply use '''__('String name','your-unique-name');''' to return the translation or '''_e('String name','your-unique-name');''' to echo the translation.

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]].

= Plugin Development Suggestions =

This last section contains some random suggestions regarding Plugin development. 

* The code of a WordPress Plugin should follow the [[WordPress Coding Standards]]. Please consider the [[Inline Documentation]] Standards as well.
* All the functions in your Plugin need to have unique names that are different from functions in the WordPress core, other Plugins, and themes. For that reason, it is a good idea to use a unique function name prefix on all of your Plugin's functions. Another possibility is to define your Plugin functions inside a class (which also needs to have a unique name).
* Do not hardcode the WordPress database table prefix (usually wp_) into your Plugins.  Be sure to use the tt$wpdb-prefix/tt variable instead. 
* Database reading is cheap, but writing is expensive. Databases are exceptionally good at fetching data and giving it to you, and these operations are (usually) lightning quick.  Making changes to the database, though, is a more complex process, and computationally more expensive.  As a result, try to minimize the amount of emwriting/em you do to the database.  Get everything prepared in your code first, so that you can make only those write operations that you need.
* SELECT only what you need. Even though databases fetch data blindingly fast, you should still try to reduce the load on the database by only selecting that data which you need to use.  If you need to count the number of rows in a table don't ttSELECT * FROM/tt, because all the data in all the rows will be pulled, wasting memory.  Likewise, if you only need the post_id and the post_author in your Plugin, then just ttSELECT/tt those specific fields, to minimize database load. Remember: hundreds of other processes may be hitting the database at the same time.  The database and server each have only so many resources to spread around amongst all those processes.  Learning how to minimize your Plugin's hit against the database will ensure that your Plugin isn't the one that is blamed for abuse of resources.
* Eliminate PHP errors in your plugin. Add ttdefine('WP_DEBUG', true);/tt to your wp-config.php file, try all of your plugin functionality, and check to see if there are any errors or warnings. Fix any that occur, and continue in debug mode until they have all been eliminated.
* Try not to echo script and style tags directly - instead use the recommended [http://codex.wordpress.org/Function_Reference/wp_enqueue_style wp_enqueue_style()] and [http://codex.wordpress.org/Function_Reference/wp_enqueue_script wp_enqueue_script()] functions. They help eliminate including duplicate scripts and styles as well as introduce dependency support. See posts by the following people for more info: [http://planetozh.com/blog/2008/04/how-to-load-javascript-with-your-wordpress-plugin/ Ozh Richardson], [http://beerpla.net/2010/01/13/wordpress-plugin-development-how-to-include-css-and-javascript-conditionally-and-only-when-needed-by-the-posts/ Artem Russakovskii], and [http://www.prelovac.com/vladimir/best-practice-for-adding-javascript-code-to-wordpress-plugin Vladimir Prelovac].

= External Resources =
* [http://planetozh.com/blog/2009/09/top-10-most-common-coding-mistakes-in-wordpress-plugins/ Top 10 Most Common Coding Mistakes in WordPress Plugins] (11SEP09)
* [http://markjaquith.wordpress.com/2006/06/02/wordpress-203-nonces/ WordPress 2.0.3: Nonces (Secure your forms with nonces)] (02JUN06)
* [http://amiworks.co.in/talk/simplified-ajax-for-wordpress-plugin-developers-using-jquery/ Simplified AJAX For WordPress Plugin Developers using Jquery](10APR08)
* [http://www.rafaeldohms.com.br/2008/03/10/desenvolvendo-plugins-para-wordpress/pt/ Desenvolvendo Plugins para WordPress by Rafael Dohms (in Brazilian Portuguese)] (10MAR08)
* [http://www.devlounge.net/extras/how-to-write-a-wordpress-plugin 12 part How to Write a Wordpress Plugin at DevLounge.net] by [http://ronalfy.com Ronald Huereca] ([http://www.devlounge.net/publik/Devlounge%20-%20How%20to%20Write%20a%20Wordpress%20Plugin.pdf PDF])
* [http://ditio.net/2007/08/09/how-to-create-wordpress-plugin-from-a-scratch/ How to create WordPress Plugin from a scratch] (9AUG07)
* [http://www.devlounge.net/articles/using-ajax-with-your-wordpress-plugin Using AJAX with your WordPress Plugin], also at DevLounce.net (25MAY07)
* [http://atd.agranite.com/emerald-coast/internet/wp-hacks/ How to Write a Simple WordPress Plugin at ATD] (22FEB05)
* [http://blog.rswr.net/2009/04/13/wordpress-plugin-saving-options/ WordPress Plugin Creation “Saving Options”] (13APR09)
* [http://mitcho.com/code/hookpress/ HookPress], a plugin that enables extending WordPress in languages other than PHP via webhooks. (26SEP09)
* [http://beerpla.net/2010/01/13/wordpress-plugin-development-how-to-include-css-and-javascript-conditionally-and-only-when-needed-by-the-posts/ How To Include CSS and JavaScript Conditionally And Only When Needed By The Posts] (13JAN10)
* [http://aaron.jorb.in/2010/wordpress-external-cron-plugin/ Demonstrating how to use the Settings API, WP_Http, and Pseudo-cron] (01MAR10)

[[Category:Plugins]]
[[Category:WordPress Development]]