This document provides an introduction to plugin development in WordPress. It discusses best practices for creating plugins, including placing code in plugin files rather than theme files and adding required headers. It also covers releasing plugins on WordPress.org and using mu-plugins, as well as providing a real-world example of a plugin.

1. Intro To Plugin
Development
Emptying out functions.php
Intro To Plugin Development
Topher DeRosia
@topher1kenobe

2. Hi, my name is Topher
I’m a WordPress developer from
Grand Rapids MI
@topher1kenobe
Intro To Plugin Development
Topher DeRosia
@topher1kenobe

3. “Just put this code in your theme’s
functions.php file…”
JUST SAY NO
Intro To Plugin Development
Topher DeRosia
@topher1kenobe

4. Plugins are packages of code that
affect your site’s functionality.
Themes are packages of code that
affect your site’s design.
DO NOT MIX.
Intro To Plugin Development
Topher DeRosia
@topher1kenobe

5. Plugins are either single files or
folders in /wp-content/plugins
Intro To Plugin Development
Topher DeRosia
@topher1kenobe

6. Typically inside each plugin folder is a file
with the same name as the folder, plus any
other files it needs.
Intro To Plugin Development
Topher DeRosia
@topher1kenobe

7. In the top of every main plugin file is a header
block with information about the plugin.
Intro To Plugin Development
Topher DeRosia
@topher1kenobe

8. The only option absolutely required is the
“Plugin Name”. The others are merely
strongly recommended.
VERY strongly recommended:
License: GNU General Public License v2 or later
License URI: http://www.gnu.org/licenses/gpl-2.0.html
Tags: custom-header, custom-menu, editor-style,
featured-images (etc)
Intro To Plugin Development
Topher DeRosia
@topher1kenobe

9. The Secret Sauce:
Any code you might have put into
functions.php in your theme could
go into a plugin.*
*with a few exceptions
Intro To Plugin Development
Topher DeRosia
@topher1kenobe

10. Example:
<?php
/*
Plugin Name: Topher’s Little Plugin
Description: Does things I want to my site
Version: 1.0
Author: Topher
*/
// 220 pixels wide by 180 pixels tall, hard crop mode
add_image_size( 'custom-size', 220, 180, true );
?> Intro To Plugin Development
Topher DeRosia
@topher1kenobe

11. Longer Example:
<?php
/*
Plugin Name: Topher’s Little Plugin
Description: Does things I want to my site
Version: 1.0
Author: Topher
*/
// 220 pixels wide by 180 pixels tall, hard crop mode
add_image_size( 'custom-size', 220, 180, true );
// Make shortcodes work inside text widgets
add_filter('widget_text', 'do_shortcode');
// Fix styling in old content
function phlog_scripts() {
wp_enqueue_style( 'phlog_styles', plugins_url( '/css/phlog.css' , __FILE__ ) );
}
add_action( 'wp_enqueue_scripts', 'phlog_scripts' );
?> Intro To Plugin Development
Topher DeRosia
@topher1kenobe

12. Explanation:
function phlog_scripts() {
wp_enqueue_style( 'phlog_styles', plugins_url( '/css/phlog.css' , __FILE__ ) );
}
add_action( 'wp_enqueue_scripts', 'phlog_scripts' );
I made a function called `phlog_scripts()`
Inside it is a function that properly enqueues a CSS file.
wp_enqueue_style takes 2 arguments, a name I made up and the path
to the CSS file.
The path to the CSS file is determined with the WordPress function
plugins_url().
Intro To Plugin Development
Topher DeRosia
@topher1kenobe

13. Releasing a Plugin
A plugin built for release on WordPress.org must meet a list of
requirements. The requirements are listed at
https://developer.wordpress.org/plugins/wordpress-org/
Best practices are found in the WordPress Plugin Handbook
https://developer.wordpress.org/plugins/
Intro To Plugin Development
Topher DeRosia
@topher1kenobe

14. mu-plugins: mu == must use
Plugins that are stored in mu-plugins are automatically activated, and
cannot be deactivated. The main file of a plugin must be stored
directly in the /mu-plugins/ directory, OR be included.
Intro To Plugin Development
Topher DeRosia
@topher1kenobe

15. mu-plugins
Intro To Plugin Development
Topher DeRosia
@topher1kenobe

16. mu-plugins
To convert a regular plugin to mu-plugins, put it in the /mu-plugins
directory and then make a new, single-file plugin that looks like this:
<?php
/*
Plugin Name: mu-plugins inclusion plugin
Description: Includes the main files of any plugins that are in mu-plugins
Author: Topher
Version: 1.0
*/
// include widget_logic
include WP_CONTENT_DIR . '/mu-plugins/widget-logic/widget_logic.php';
?>
Intro To Plugin Development
Topher DeRosia
@topher1kenobe

17. Extra Credit: WP-CLI
WP-CLI is the command line tool for WordPress. It can create an
empty plugin with the proper header in place:
wp scaffold plugin --prompt
This will ask questions like this:
1/4 <slug>: topher-test
2/4 [--plugin_name=<title>]: Topher's Test
3/4 [--skip-tests] (Y/n): Y
4/4 [--activate] (Y/n): Y
Success: Created /home/topher1/topher1kenobe.com/wp-content/plugins/topher-test
1/1 <plugin>: topher-test
Success: Created test files.
Intro To Plugin Development
Topher DeRosia
@topher1kenobe

18. Real World Example
Easy Digital Downloads - Customer Contact
Intro To Plugin Development
Topher DeRosia
@topher1kenobe

19. Easy Digital Downloads - Customer Contact
<?php
/*
Plugin Name: Easy Digital Downloads - Customer Contact
Version: 1.0
Description: Creates a tab on the Customer page for sending an email to that customer.
Author: Topher
Author URI: http://topher1kenobe.com
Plugin URI: http://topher1kenobe.com
Text Domain: edd-customer-contact
Domain Path: /languages
License: GNU General Public License v2 or later
License URI: http://www.gnu.org/licenses/gpl-2.0.html
*/
Intro To Plugin Development
Topher DeRosia
@topher1kenobe

20. Easy Digital Downloads - Customer Contact
Comment everything!
Docblock
/**
* Contact a customer
*
* @since 2.3
* @param array $args The $_POST array being passeed
* @return int Whether it was a successful email
*/
Inline
// make sure we're allowed to be here at all
if ( ! is_admin() || ! current_user_can( $customer_edit_role ) ) {
wp_die( __( 'You do not have permission to contact this customer.', 'edd' ) );
} Intro To Plugin Development
Topher DeRosia
@topher1kenobe

21. Easy Digital Downloads - Customer Contact
Insert Code Review Here
Intro To Plugin Development
Topher DeRosia
@topher1kenobe

22. Easy Digital Downloads - Customer Contact
Get Help
From other developers (Slack, Stack Overflow, friends)
From parent plugin developer (if making an Addon they probably want to
help)
From documentation (https://developer.wordpress.org/plugins/)
From code (read through plugins doing similar things)
Intro To Plugin Development
Topher DeRosia
@topher1kenobe

23. THANKS FOR
LISTENING
Intro To Plugin Development
Topher DeRosia
@topher1kenobe
http://topher1kenobe.com
Follow me @topher1kenobe