Creating Your First WordPress Plugin

Creating Your First
WordPress Plugin

Brad Williams
@williamsba

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

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

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.

What is a Plugin?

http://wordpress.org/extend/plugins/

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…

Plugin Header Requirements
<?php
/*
Plugin Name: WordCamp Philly 2011
Plugin URI: http://webdevstudios.com/support/wordpress-plugins/
Description: Plugin for WordCamp Philly
Version: 1.0
Author: Brad Williams
Author URI: http://webdevstudios.com
*/
?>

Plugin is now available to be
activated in WordPress!

Sanity Practices
Prefix Everything!
update_option() //BAD FUNCTION NAME 

bw_wcphilly_update_option() //GOOD FUNCTION NAME! 

$settings //BAD VARIABLE NAME 

$bw_wcphilly_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
• /js can make it much easier to
• /css track the flow of your plugin over
• /includes time.
• /images

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

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

Important Techniques
Plugin Activation Function
<?php
register_activation_hook( __FILE__, 'bw_wcphilly_install' );

function bw_wcphilly_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

Plugin Deactivation Function
<?php
register_deactivation_hook( __FILE__, 'bw_wcphilly_deactivate' );

function bw_wcphilly_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

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_wcphilly_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/

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/

Hooks
Action Hooks
Enables you to execute a function at specific points in the WordPress loading process

<?php
add_action( 'user_register', 'bw_wcphilly_welcome_email' );

function bw_wcphilly_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

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

Hooks
Filter Hooks
Enables you to manipulate the output of code and content in WordPress

<?php
add_filter( 'the_content', 'bw_wcphilly_filter_content' );

function bw_wcphilly_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

Hooks
Filter Hooks
<?php
add_filter ( 'the_content', „bw_wcphilly_insertFootNote' );

function bw_wcphilly_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;
}
?>


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


Hooks
Number of Hooks in WordPress by Version

http://adambrown.info/p/wp_hooks

Plugin Foundation

It‟s important to start with a solid foundation

Plugin Foundation
Example Plugin Foundation
<?php
/*
Plugin Name: WordCamp Philly
Plugin URI: http://webdevstudios.com/support/wordpress-plugins/
Description: Plugin for WordCamp Philly
Version: 1.0
Author: Brad Williams
Author URI: http://webdevstudios.com
License: GPLv2
*/

// DEFINE CONSTANTS
define( „WCPHILLY_VERSION', '1.0' );
define( 'WCPHILLY_TEXTDOMAIN', „wcphilly_plugin' );
define( 'WCPHILLY_BASENAME', plugin_basename(__FILE__) );
define( 'WCPHILLY_DIR', plugin_dir_path( __FILE__ ) );
define( 'WCPHILLY_URL', plugins_url( 'my-new-plugin/' ) );

require_once( WCPHILLY_DIR . 'includes/core.php' );
?>

Shortcode Example
Example
<?php
// Register a new shortcode: [book]
add_shortcode( 'book', 'bw_wcphilly_book' );

// The callback function that will replace [book]
function bw_wcphilly_book() {
return '<a href="http://amzn.to/plugindevbook">Professional WordPress Plugin
Development</a>';
}
?>

Menu Example
Example Custom Menu
<?php
add_action( 'admin_menu', 'bw_wcphilly_create_menu' );

function bw_wcphilly_create_menu() {

//create custom top-level menu
add_menu_page('WC Philly Settings Page', 'WC Philly',
'manage_options', 'wcphilly-plugin', 'bw_wcphilly_settings_page' );

//create submenu items
add_submenu_page( 'wcphilly-plugin', 'About My Plugin', 'About',
'manage_options', 'wcphilly-plugin_about', 'bw_wcphilly_about_page' );
add_submenu_page( 'wcphilly-plugin', 'Help with My Plugin', 'Help',
'manage_options', 'wcphilly-plugin_help', 'bw_wcphilly_help_page' );
add_submenu_page( 'wcphilly-plugin', 'Uinstall My Plugin', 'Uninstall',
'manage_options', 'nyc-meetup-plugin_uninstall', 'bw_wcphilly_uninstall_page' );

}
?>

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
$menu_title – The submenu name
$capability – Capability required to view submenu

Menu Example
Add Submenu to Custom Post Type
<?php


//create submenu items
add_submenu_page( 'edit.php?post_type=movies', 'About My Plugin', 'About',
'manage_options', „wcphilly-plugin_about', 'bw_wcphilly_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

Menu Example
Add Submenu to Existing Menu
<?php


//create Settings submenu page
add_options_page('WC Philly Settings Page', 'WC Philly', 'manage_options',
'wcphilly-plugin', 'bw_wcphilly_settings_page' );

}

function bw_wcphilly_settings_page() {
//settings page
}?>

Menu Example
add_options_page()
Parameters:
$menu_title – The menu name
$capability - Capability required to view menu

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()

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

Contact

Brad Williams
brad@webdevstudios.com

Blog: strangework.com
Twitter: @williamsba
IRC: WDS-Brad

http://www.slideshare.net/williamsba

Creating Your First WordPress Plugin

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