Brad Williams, the co-author of Professional WordPress Plugin Development, gives his presentation on Intro to WordPress Plugin Development to the NYC WordPress Meetup group in March 2011.

1. Intro to WordPress
Plugin Development
Brad Williams
@williamsba

2. Who Am I?
Brad Williams
Co-Founder of WebDevStudios.com
Organizer Philly WordPress Meetup
& WordCamp Philly
Co-Author of Professional WordPress
(http://bit.ly/pro-wp)
& Professional WordPress Plugin Development
(http://amzn.to/plugindevbook)
Slides available at: http://www.slideshare.net/williamsba

3. Topics
 What is a Plugin?
 Types of Plugins
 Sanity Practices and Plugin Foundation
 Determining Paths in a Plugin
 Activation, Deactivation, and Uninstall Methods
 Shortcodes, Menus, and Settings Overview
 Hooks: Actions and Filters Explained
 Resources for Plugin Developers

4. What is a Plugin?
A plugin in WordPress is a PHP script
that extends, or alters, the core
functionality of WordPress.
Quite simply plugins are files installed in
WordPress to add a feature, or set of
features, to WordPress.

5. What is a Plugin?
http://wordpress.org/extend/plugins/

6. Types and Statuses of Plugins
 Active – Plugin is active and running in WordPress
 Inactive – Plugin is installed but not active. No code from the plugin is
executed
 Must-Use – All plugins installed in wp-content/mu-plugins. All plugins
are loaded automatically. Only way to deactivate is to remove.
 Drop-ins – Core functionality of WordPress can be replaced by Drop-in
plugins.
 advanced-cache-php – Advanced caching plugin
 db.php – Custom database class
 maintenance.php – Custom maintenance message
 sunrise.php – Domain mapping
 And more…

7. Plugin Header Requirements
<?php
/*
Plugin Name: WordPress NYC Meetup
Plugin URI: http://webdevstudios.com/support/wordpress-plugins/
Description: Plugin for the WordPress NYC Meetup
Version: 1.0
Author: Brad Williams
Author URI: http://webdevstudios.com
License: GPLv2
*/
?>
Plugin is now available to be
activated in WordPress!

8. Sanity Practices
Prefix Everything!
update_option() //BAD FUNCTION NAME 
bw_nycmeetup_update_option() //GOOD FUNCTION NAME! 
$settings //BAD VARIABLE NAME 
$bw_nycmeetup_settings //GOOD VARIABLE NAME! 
Organized Folder Structure
• /unique-plugin-name
• unique-plugin-name.php Keeping your files organized
• uninstall.php using a clean folder structure can
• /js make it much easier to track the
• /css flow of your plugin over time.
• /includes
• /images

9. Determining Paths
Local Paths
<?php
//display local path to my plugin directory
echo plugin_dir_path( __FILE__ );
?>
Would display: /public_html/wp-content/plugins/my-new-plugin/
<?php
//display local path to my includes/functions.php file
echo plugin_dir_path( __FILE__ ) .’includes/functions.php’;
?>
Would display: /public_html/wp-content/plugins/my-new-plugin/includes/functions.php
__FILE__ is a “magical” PHP
constant containing the full path
and filename of the file

10. Determining Paths
URL Paths
<?php
//display the URL to images/icon.png
echo plugins_url( 'images/icon.png', __FILE__ );
?>
Would display: http://example.com/wp-content/plugins/my-new-plugin/images/icon.png
Advantages of plugins_url()
• Supports the mu-plugins directory
• Auto detects SSL, so if enabled the URL would return https
• Uses the WP_PLUGIN_URL constant, meaning it can detect the correct path even if
/wp-content has been moved
• Supports Multisite using the WPMU_PLUGIN_URL constant
Plugin is now available to be
activated in WordPress!

11. Important Techniques
Plugin Activation Function
<?php
register_activation_hook( __FILE__, 'bw_nycmeetup_install' );
function bw_nycmeetup_install() {
If ( version_compare( get_bloginfo( 'version' ), ‘4.0', '<' ) ) {
deactivate_plugins( plugin_basename( __FILE__ ) ); // Deactivate our plugin
wp_die( 'This plugin requires WordPress version 4.0 or higher.' );
}
}
?>
register_activation_hook( $file, $function )
Parameters:
• $file (string) (required) – Path to the primary plugin file
• $function (string) (required) – Function to be executed when plugin is
activated
http://codex.wordpress.org/Function_Reference/register_activation_hook

12. Important Techniques
Plugin Deactivation Function
<?php
register_deactivation_hook( __FILE__, 'bw_nycmeetup_deactivate' );
function bw_nycmeetup_deactivate() {
//do stuff
}?>
register_deactivation_hook( $file, $function )
Parameters:
• $file (string) (required) – Path to the primary plugin file
• $function (string) (required) – Function to be executed when plugin is
deactivated
REMEMBER: Deactivating is NOT uninstalling. Always
assume your users will reactivate at a later date.
http://codex.wordpress.org/Function_Reference/register_deactivation_hook

13. Important Techniques
Plugin Uninstall
1. Create a uninstall.php file in the root directory of your plugin
2. Add all uninstall code to this file
<?php
if( !defined( 'WP_UNINSTALL_PLUGIN' ) )
exit ();
// Delete option from options table
delete_option( ‘bw_nycmeetup_options' );
?>
If the WP_UNINSTALL_PLUGIN constant is not
defined we know WordPress did not call this file.
This is a security measure in WordPress
Uninstall.php is the recommended uninstall method, but
there is another method called register_uninstall_hook()
http://jacobsantos.com/2008/general/wordpress-27-plugin-uninstall-methods/

14. What is a Hook?
Hooks enable plugin developers to “hook”
into WordPress and change how it works
without modifying the core code
There are two kinds of hooks: Actions and Filters
http://jacobsantos.com/2008/general/wordpress-27-plugin-uninstall-methods/

15. Hooks
Action Hooks
Enables you to execute a function at specific points in the WordPress loading process
<?php
add_action( 'user_register', 'bw_nycmeetup_welcome_email' );
function bw_nycmeetup_welcome_email( $user_id ) {
$user_info = get_userdata( $user_id );
//populate email values
$email_to = is_email( $user_info->user_email );
$email_subject = 'Welcome!';
$email_msg = 'Thank you for registering on my website!';
//send welcome email
wp_mail( $email_to, $email_subject, $email_msg );
}
?>
http://codex.wordpress.org/Plugin_API/Action_Reference

16. Hooks
Common Action Hooks
• plugins_loaded – Earliest hook in the WP loading process, after all plugins have
been loaded
• init – Fire after most of WP is set up so all information is available
admin_menu – Runs when the basic admin menu structure is in place
• template_redirect – Executed just before the theme template is chosen
• wp_head – Executed on the front end of WordPress between the <head> tags
• wp_footer – Runs in the footer of your theme
• admin_head – Executed on the admin side of WordPress between the <head>
tags
• admin_footer – Runs in the footer of the admin side of WordPress
• user_register – Executes when a new user account is created
• save_post – Runs when a post or page is created or updated
http://codex.wordpress.org/Plugin_API/Action_Reference

17. Hooks
Filter Hooks
Enables you to manipulate the output of code and content in WordPress
<?php
add_filter( 'the_content', 'bw_nycmeetup_filter_content' );
function bw_nycmeetup_filter_content( $text ) {
$text = str_replace( 'Drupal', 'WordPress', $text );
return $text;
}
?>
Change every instance of Drupal to WordPress in your content >:)
REMEMBER: Using a filter does NOT change the
content in the database, it simply alters it prior to
displaying
http://codex.wordpress.org/Plugin_API/Filter_Reference

18. Hooks
Filter Hooks
<?php
add_filter ( 'the_content', 'insertFootNote' );
function insertFootNote( $content ) {
if( !is_feed() && !is_home() ) {
$content .= "<div class='subscribe'>";
$content .= "<h4>Enjoyed this article?</h4>";
$content .= "<p>Subscribe to my <a href='http://feeds2.feedburner.com/strangework'>RSS
feed</a>!</p>";
$content .= "</div>";
}
return $content;
}
?>
http://codex.wordpress.org/Plugin_API/Filter_Reference

19. Hooks
Common Filter Hooks
• the_content – Filter applied to the content of the post or page
• the_title – Applied to the post title
• body_class – Applied to the <body> tag class parameter
• default_content – Applied to the content on a new post or page
• comment_text – Applied to the comment text of a comment
http://codex.wordpress.org/Plugin_API/Filter_Reference

20. Hooks
Number of Hooks in WordPress by Version
http://adambrown.info/p/wp_hooks

21. Plugin Foundation
It’s important to start with a solid foundation

22. Plugin Foundation
Example Plugin Foundation
<?php
/*
Plugin Name: WordPress NYC Meetup
Plugin URI: http://webdevstudios.com/support/wordpress-plugins/
Description: Plugin for the WordPress NYC Meetup
Version: 1.0
Author: Brad Williams
Author URI: http://webdevstudios.com
License: GPLv2
*/
// DEFINE CONSTANTS
define( 'BWNYCMEETUP_VERSION', '1.0' );
define( 'BWNYCMEETUP_TEXTDOMAIN', 'bwnycmeetup_plugin' );
define( 'BWNYCMEETUP_BASENAME', plugin_basename(__FILE__) );
define( 'BWNYCMEETUP_DIR', plugin_dir_path( __FILE__ ) );
define( 'BWNYCMEETUP_URL', plugins_url( 'my-new-plugin/' ) );
require_once( BWNYCMEETUP_DIR . 'includes/core.php' );
?>

23. Shortcode Example
Example
<?php
// Register a new shortcode: [book]
add_shortcode( 'book', 'bw_nycmeetup_book' );
// The callback function that will replace [book]
function bw_nycmeetup_book() {
return '<a href="http://amzn.to/plugindevbook">Professional WordPress Plugin
Development</a>';
}
?>

24. Menu Example
Example Custom Menu
<?php
add_action( 'admin_menu', 'bw_nycmeetup_create_menu' );
function bw_nycmeetup_create_menu() {
//create custom top-level menu
add_menu_page( 'NYC Meetup Settings Page', 'NYC Meetup',
'manage_options', 'nyc-meetup-plugin', 'bw_nycmeetup_settings_page' );
//create submenu items
add_submenu_page( 'nyc-meetup-plugin', 'About My Plugin', 'About',
'manage_options', 'nyc-meetup-plugin_about', 'bw_nycmeetup_about_page' );
add_submenu_page( 'nyc-meetup-plugin', 'Help with My Plugin', 'Help',
'manage_options', 'nyc-meetup-plugin_help', 'bw_nycmeetup_help_page' );
add_submenu_page( 'nyc-meetup-plugin', 'Uinstall My Plugin', 'Uninstall',
'manage_options', 'nyc-meetup-plugin_uninstall', 'bw_nycmeetup_uninstall_page' );
}
?>

25. Menu Example
add_menu_page()
Parameters:
$page_title – The title tag text
$menu_title – The menu name
$capability - Capability required to view menu
$menu_slug – Unique slug to reference menu by
$function – Function that displays the pages content
$icon_url – URL for a custom menu icon
$position – Position the menu should appear
add_submenu_page()
Parameters:
$parent_slug – Slug name for the parent menu
$page_title – The title tag text
$menu_title – The submenu name
$capability – Capability required to view submenu
$menu_slug – Unique slug to reference menu by
$function – Function that displays the pages content

26. Menu Example
Add Submenu to Custom Post Type
<?php
add_action( 'admin_menu', 'bw_nycmeetup_create_menu' );
function bw_nycmeetup_create_menu() {
//create submenu items
add_submenu_page( 'edit.php?post_type=movies', 'About My Plugin', 'About',
'manage_options', 'nyc-meetup-plugin_about', 'bw_nycmeetup_about_page' );
}
?>
You can easily add a submenu to any Custom Post
Type menu by setting the $parent_slug to edit.php?
post_type=POSTTYPENAME

27. Menu Example
Add Submenu to Existing Menu
<?php
add_action( 'admin_menu', 'bw_nycmeetup_create_menu' );
function bw_nycmeetup_create_menu() {
//create Settings submenu page
add_options_page('NYC Meetup Settings Page', 'NYC Meetup',
'manage_options', 'nyc-meetup-plugin', 'bw_nycmeetup_settings_page' );
}
function bw_nycmeetup_settings_page() {
//settings page
}
?>

28. Menu Example
add_options_page()
Parameters:
$page_title – The title tag text
$menu_title – The menu name
$capability - Capability required to view menu
$menu_slug – Unique slug to reference menu by
$function – Function that displays the pages content
add_dashboard_page()
add_posts_page()
add_media_page()
add_links_page()
add_pages_page()
add_comments_page()
add_theme_page()
add_plugins_page()
add_users_page()
add_management_page()
add_options_page()

29. Basic Settings Page Example
Register your settings
add_action('admin_init', 'bw_nycmeetup_register_settings');
// WordPress Settings API
function bw_nycmeetup_register_settings(){
register_setting( 'bw_nycmeetup_settings_group', 'bw_nycmeetup_settings',
'bw_nycmeetup_settings_validate' );
}
register_setting()
Parameters:
$option_group – A unique settings group name
$option_name – A unique option name for the group
$sanitize_callback – Callback function to sanitize the option's
values

30. Menu Example
add_menu_page()
Parameters:
$page_title – The title tag text
$menu_title – The menu name
$capability - Capability required to view menu
$menu_slug – Unique slug to reference menu by
$function – Function that displays the pages content
$icon_url – URL for a custom menu icon
$position – Position the menu should appear
add_submenu_page()
Parameters:
$parent_slug – Slug name for the parent menu
$page_title – The title tag text
$menu_title – The submenu name
$capability – Capability required to view submenu
$menu_slug – Unique slug to reference menu by
$function – Function that displays the pages content

31. Basic Settings Page Example
// Plugin settings page
function bw_nycmeetup_settings_page() { ?>
<div class="wrap">
<div class="icon32" id="icon-options-general"><br /></div>
<h2>My Plugin Settings</h2>
<form method="post" action="options.php">
<?php settings_fields( 'bw_nycmeetup_settings_group' ); ?>
<?php $options = get_option( 'bw_nycmeetup_settings' ); ?>
<table class="form-table">
<tr valign="top"><th scope="row">Name:</th>
<td><input name="bw_nycmeetup_settings[name]" type="text" value="<?php echo esc_attr( $options['name'] ); ?>"
/></td>
</tr>
<tr valign="top"><th scope="row">Favorite Color:</th>
<td>
<select name="bw_nycmeetup_settings[color]">
<option value="orange" <?php selected( $options['color'], 'orange' ); ?>>Orange</option>
<option value="black" <?php selected( $options['color'], 'black' ); ?>>Black</option>
</select>
</td>
</tr>
<tr valign="top"><th scope="row">Enable Rage Mode?</th>
<td><input type="checkbox" name="bw_nycmeetup_settings[rage]" <?php if ( isset( $options['rage'] ) )
checked( $options['rage'], 'on' ); ?>/>
</td>
</tr>
</table>
<p class="submit">
<input type="submit" class="button-primary" value="Save Settings" />
</p>
</form>
</div>
<?php }

32. Basic Settings Page Example
Register your settings
// Validation
function bw_nycmeetup_settings_validate($input) {
$input['name'] = strip_tags( $input['name'] );
$input['color'] = strip_tags( $input['color'] );
$input['rage'] = ( isset( $input['rage'] ) ) ? strip_tags( $input['rage'] ) : null;
return $input;
}

33. Basic Settings Page Example
Now you have a settings page!

34. Plugin Developer Resources
 Official Resources
› WordPress Core!
› http://wordpress.org/extend/plugins/
› http://codex.wordpress.org/Writing_a_Plugin
› http://codex.wordpress.org/Data_Validation
› http://wordpress.org/support/forum/hacks
› http://lists.automattic.com/mailman/listinfo/wp-hackers
› http://codex.wordpress.org/IRC ( #wordpress channel
› http://wpdevel.wordpress.com/
› http://wordpress.org/extend/ideas/
 Developer Websites
› http://wpengineer.com
› http://phpxref.ftwr.co.uk/wordpress/
› http://adambrown.info/p/wp_hooks

35. Contact
Brad Williams
brad@webdevstudios.com
Blog: strangework.com
Twitter: @williamsba
IRC: WDS-Brad
http://www.slideshare.net/williamsba