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