This document provides an introduction to building WordPress plugins, including: - What a WordPress plugin is and examples of simple to advanced plugins - The basic structure of a plugin including the required plugin header - Important plugin building blocks like PHP functions, hooks, templates, and APIs - How to add functionality using actions, filters, and enqueueing assets - Saving plugin data using options like the Options API - Example tasks for readers to create a simple lyrics plugin that filters post content and includes a settings page

1. Extending WordPress
A (developers) guide to building your
first WordPress plugin
@jon_bossenger http://jonathanbossenger.com
http://jonathanbossenger.com/extending-wordpress/

2. WHAT IS A WORDPRESS PLUGIN?
• One (or more) files (PHP, HTML, CSS, JavaScript)
• Extend WordPress functionality
• Simple – display 10 most recent posts
• Advanced – ticketing/eCommerce

3. PLUGIN BASICS
• Single file
• More complex structure
– PHP
– HTML/CSS (templates)
– JavaScript
• Plugin Header

4. PLUGIN HEADER
/*
Plugin Name: My Toolset
Plugin URI: http://URI_Of_Page_Describing_Plugin_and_Updates
Description: This describes my plugin in a short sentence
Version: 1.5
Author: John Smith
Author URI: http://URI_Of_The_Plugin_Author
License: GPL2
License URI: https://www.gnu.org/licenses/gpl-2.0.html
Domain Path: /languages
Text Domain: my-toolset
*/

5. TASK
• Create a lyrics plugin
• Copy and edit the plugin header from
hello.php to suit your requirements
• The plugin should be available in your plugins
list, in WP admin

6. PLUGIN BUILDING BLOCKS
• PHP Functions
• WordPress Action Hooks
• WordPress Filter Hooks
• Templates
• WordPress APIs (eg Options, Settings API)
• CSS/JavaScript (wp_enqueue_script)

7. It starts with an idea!
• At the end of each post, show a line from a
song
• Make the line stand out somehow
• (Bonus) Make it possible to edit the song lyrics

8. Hooks
• A pre existing event, that triggers either an
Action or a applies a Filter
• Action Hooks
• Filter Hooks

9. Actions
• A PHP function that is executed at a specific
point (hook)
• Create custom Actions or remove code from
an existing Action

10. Filters
• A function that is associated with an
existing Action
• Create custom Filters to replace code from an
existing Action.
• Filters allow you to replace or alter specific
data.

11. Examples
// Action Hook Example
function my_action_hook_function(){
// do some things here
}
add_action(‘some_action_hook’, ‘my_action_hook_function’);
// Filter Hook Example
function my_filter_hook_function($some_variable){
// do something with $some_variable here
return $some_variable;
}
add_filter( ‘some_filter_hook’, ‘my_filter_hook_function’ );

12. Building our plugin
• ‘the_content’ filter hook, hooks into the
‘the_content’ filter, which is used to filter the
content of the post after it is retrieved from
the database and before it is printed to the
screen.
• add_filter(‘the_content’, ‘my_filter_function’)

13. Building our plugin (cont)
• ‘wp_enqueue_scripts’ action hook, used to
enqueue assets that are used on the front end
(eg CSS, JavaScript)
• add_action(‘wp_enqueue_scripts’,
‘my_enqueue_scripts_function’);
• ‘wp_enqueue_style’ function, used to
enqueue a style sheet file (load it on the front
end)
• wp_enqueue_style( $handle, $source);

14. TASK
• Create a custom filter hook to add one line of text
AFTER the post content
• Create a custom action hook to load the custom CSS
file
• Add some CSS styling to your one line of text (you did
wrap it in a div or p, didn’t you?)
• TIP: $plugin_url = plugin_dir_url( __FILE__ );

15. TASK
• Using the hello.php ‘hello_dolly_get_lyric’
function, create a function that gets one line
from the song to use in your custom
‘the_content’ filter in the previous step
• It should randomly select a lyric to display

16. Saving Plugin Data
• Options API
• Custom Tables
• Settings API
• Adding menus

17. Building our plugin (cont)
• ‘admin_menu’ action hook – used to add
admin menu items
• ‘add_options_page’ function – Add sub menu
page to the Settings menu
• add_options_page( $page_title, $menu_title,
• $capability, $menu_slug, $function);

18. Building our plugin (cont)
• add_option($option, $value) – add an option
to the options table
• update_option($option, $value) – update an
option by its key
• get_option($option, $default) – get an option
value

19. Bonus Task
• Create a Settings menu item for your plugin
• Create a simple form that allows you to save
new song lyrics as a WP option
• Modify your plugin to use these saved lyrics
instead of ‘hello dolly’

20. Development Tips and Tricks
• WP_DEBUG
• DebugBar
• BugFu
• PHPStorm

21. Resources
• Google ;-)
• https://codex.wordpress.org
• https://developer.wordpress.org/
• https://developer.wordpress.org/plugins/
• https://codex.wordpress.org/WordPress_Coding_Standards

22. Resources (cont)
• https://webdevstudios.com/books/professional-
wordpress-plugin-development/
• https://github.com/hlashbrooke/WordPress-
Plugin-Template
• WPSA Slack

23. Questions ?