Presented at WordCamp Montreal 2017 For many WordPress users, even seasoned PHP developers, creating new plugins for WordPress seems like a daunting task. This presentation aims to show attendees how simple creating plugins for WordPress from the ground up can be by looking at the architecture of a WordPress plugin, from the basic concepts of registering actions and filters to more advanced concepts such as the creation of admin pages and registering shortcodes.

1. Yannick Lefebvre
@ylefebvre
ylefebvre.ca
WordPress Plugin Developer / Author
Plugin Development Demystified
Second Edition

2. Topics
●
Plugins Overview
●
Types of plugin developers
●
Plugins vs functions.php
●
File Structure
●
Required Tools
●
Plugin file header
●
Turning functions.php code
into a plugin
●
Actions hook
●
Filter hook
●
Shortcodes
●
More tools
●
Considerations
●
Publishing your plugin
●
Recommended Readings
●
Questions
Plugin Development Demystified
Slides at ylefebvre.ca/wcmtl2017

3. About me
●
WordPress user since 2004
●
Released first plugin in 2005
●
8 plugins on oficial repository
●
Author of WordPress Plugin
Development Cookbook, Second
Edition
– Use code WPDCSEd50 to get
50% of eBook on
packtpub.com until Sept 12
●
Custom plugin development
Slides at ylefebvre.ca/wcmtl2017

4. Plugins Overview
●
Extend WordPress capabilities
●
Open plugin architecture
●
Ofer function of varying complexity
●
More than 50,000 plugins available today!

5. Plugins Overview
●
Extend WordPress capabilities
●
Open plugin architecture
●
Ofer function of varying complexity
●
More than 50,000 plugins available today!
Who makes
these plugins
and what
tools do they
use to make
them?

6. Types of plugin developers
Site administrator Website developer
Community developerFreelance developer

7. Plugins vs functions.php
●
Ever written your own plugin?

8. Plugins vs functions.php
●
Ever written your own plugin?
●
Ever added code to functions.php on your own site or in a
customer project?
function ylefebvre_add_favicon(){ ?>
<link rel="shortcut icon" href="<?php echo
get_stylesheet_directory_uri(); ?>/images/favicon.ico"/>
<?php }
add_action( 'wp_head', 'ylefebvre_add_favicon' );
function ylefebvre_add_favicon(){ ?>
<link rel="shortcut icon" href="<?php echo
get_stylesheet_directory_uri(); ?>/images/favicon.ico"/>
<?php }
add_action( 'wp_head', 'ylefebvre_add_favicon' );

9. Plugins vs functions.php
●
Ever written your own plugin?
functions.php code is 99%of the way towards making a
plugin with 0%of the benefits
●
Ever added code to functions.php on your own site or in a
customer project?
function ylefebvre_add_favicon(){ ?>
<link rel="shortcut icon" href="<?php echo
get_stylesheet_directory_uri(); ?>/images/favicon.ico"/>
<?php }
add_action( 'wp_head', 'ylefebvre_add_favicon' );
function ylefebvre_add_favicon(){ ?>
<link rel="shortcut icon" href="<?php echo
get_stylesheet_directory_uri(); ?>/images/favicon.ico"/>
<?php }
add_action( 'wp_head', 'ylefebvre_add_favicon' );

10. Plugins vs functions.php
Benefit functions.php Plugin
Easily activated or deactivated
without need to search or risk of
afecting other code
Theme-independent
Easy to update in customer
installations

11. Basic Plugin File Structure
●
One or more plain text php code file(s)
●
Can contain other file types (e.g. images,
text files, translation files, etc…)
●
Distributed as zip file

12. Required Tools
Required
●
Text Editor (Notepad)

13. Required Tools
Optional
●
Code editor (e.g. Notepad++, Programmer’s
Notepad, Sublime Text)

14. Required Tools
Optional
●
CodeIDE (PhpStorm)

15. Required Tools
Optional
●
Image Editor
●
Local web server
– VVV – Varying Vagrant Vagrants
– XAMPP
– MAMP
– WSL – Windows Subsystem for Linux)
●
Archive Tool

16. Plugin File Header
<?php
/*
Plugin Name: My New Google Analytics Plugin
Plugin URI: http://ylefebvre.ca
Description: New revolutionary GA Plugin
Version: 1.0
Author: Yannick Lefebvre
Author URI: http://ylefebvre.ca
License: GPL2
*/
<?php
/*
Plugin Name: My New Google Analytics Plugin
Plugin URI: http://ylefebvre.ca
Description: New revolutionary GA Plugin
Version: 1.0
Author: Yannick Lefebvre
Author URI: http://ylefebvre.ca
License: GPL2
*/
●
Registers the plugin with WordPress
●
Data visible to users in the Plugins admin section

17. First plugin sighting

18. Turning functions.php code into a plugin
<?php
/*
Plugin Name: Add favicon
Plugin URI: http://ylefebvre.ca
Description: Add favicon to site
Version: 1.0
Author: Yannick Lefebvre
Author URI: http://ylefebvre.ca
License: GPL2
*/
function ylefebvre_add_favicon(){ ?>
<link rel="shortcut icon" href="<?php echo
get_stylesheet_directory_uri();?>/images/favicon.ico"/>
<?php }
add_action( 'wp_head', 'ylefebvre_add_favicon' );
<?php
/*
Plugin Name: Add favicon
Plugin URI: http://ylefebvre.ca
Description: Add favicon to site
Version: 1.0
Author: Yannick Lefebvre
Author URI: http://ylefebvre.ca
License: GPL2
*/
function ylefebvre_add_favicon(){ ?>
<link rel="shortcut icon" href="<?php echo
get_stylesheet_directory_uri();?>/images/favicon.ico"/>
<?php }
add_action( 'wp_head', 'ylefebvre_add_favicon' );

19. Three powerful tools
Shortcodes
Action
hooks
Filter
hooks

20. Three powerful tools
Action Hook
Do you want to do something
when an event occurs?
Filter Hook Shortcode

21. Three powerful tools
Action Hook
Do you want to do something
when an event occurs?
Event
WordPress is displaying page
header
Response
Execute plugin code to display
additional header content
Filter Hook Shortcode

22. Three powerful tools
Action Hook
Do you want to do something
when an event occurs?
Event
WordPress is displaying page
header
Response
Execute plugin code to display
additional header content
Filter Hook
Do you want to modify data
before it is displayed?
Shortcode

23. Three powerful tools
Action Hook
Do you want to do something
when an event occurs?
Event
WordPress is displaying page
header
Response
Execute plugin code to display
additional header content
Filter Hook
Do you want to modify data
before it is displayed?
Event
WordPress is preparing posts
to be displayed
Response
Execute plugin code to add
Javascript code to all links
Shortcode

24. Three powerful tools
Action Hook
Do you want to do something
when an event occurs?
Event
WordPress is displaying page
header
Response
Execute plugin code to display
additional header content
Filter Hook
Do you want to modify data
before it is displayed?
Provide easy-to-use code for
users to add content to site
Event
WordPress is preparing posts
to be displayed
Response
Execute plugin code to add
Javascript code to all links
Shortcode

25. Three powerful tools
Action Hook
Do you want to do something
when an event occurs?
Event
WordPress is displaying page
header
Response
Execute plugin code to display
additional header content
Filter Hook
Do you want to modify data
before it is displayed?
Provide easy-to-use code for
users to add content to site
Event
WordPress is preparing posts
to be displayed
Response
Execute plugin code to add
Javascript code to all links
Shortcode
Event
User has placed shortcode in
page content
Response
Generate content and send
back to WP

26. Assigning an action hook
add_action( 'hook_name', 'your_function_name',
[priority], [accepted_args] );
Example
add_action( 'wp_head', 'newga_header' );
add_action( 'hook_name', 'your_function_name',
[priority], [accepted_args] );
Example
add_action( 'wp_head', 'newga_header' );
●
Most hook names can be found in WordPress Codex or other
repositories
●
864 action hooks
●
You can learn about hooks by looking into WP source code:
function wp_head() {
do_action( 'wp_head' );
}
function wp_head() {
do_action( 'wp_head' );
}

27. Full action hook implementation
/* Header code */
add_action( 'wp_head', 'newga_header' );
function newga_header() { ?>
<script>
(function(i,s,o,g,r,a,m){i['GoogleAnalyticsObject']=r;
i[r]=i[r]||function(){
(i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();
a=s.createElement(o),
m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;
m.parentNode.insertBefore(a,m)})(window,document,'script',
'https://www.google-analytics.com/analytics.js','ga');
ga('create', 'UA-0000000-0', 'auto');
ga('send', 'pageview');
</script>
<? }
/* Header code */
add_action( 'wp_head', 'newga_header' );
function newga_header() { ?>
<script>
(function(i,s,o,g,r,a,m){i['GoogleAnalyticsObject']=r;
i[r]=i[r]||function(){
(i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();
a=s.createElement(o),
m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;
m.parentNode.insertBefore(a,m)})(window,document,'script',
'https://www.google-analytics.com/analytics.js','ga');
ga('create', 'UA-0000000-0', 'auto');
ga('send', 'pageview');
</script>
<? }

28. Full action hook implementation

29. Assigning a filter hook
add_filter( 'filter_name', 'your_function_name',
[priority], [accepted_args] );
Example
add_filter( 'the_content', 'newga_content_filter' );
add_filter( 'filter_name', 'your_function_name',
[priority], [accepted_args] );
Example
add_filter( 'the_content', 'newga_content_filter' );
●
Receive data that can be modified and returned
●
1674 filter hooks
function the_content($more_link_text = null, $stripteaser = 0)
{
$content = get_the_content($more_link_text, $stripteaser);
$content = apply_filters('the_content', $content);
$content = str_replace(']]>', ']]&gt;', $content);
echo $content;
}
function the_content($more_link_text = null, $stripteaser = 0)
{
$content = get_the_content($more_link_text, $stripteaser);
$content = apply_filters('the_content', $content);
$content = str_replace(']]>', ']]&gt;', $content);
echo $content;
}

30. Full filter hook implementation
add_filter( 'the_content', 'newga_content_filter' );
function newga_content_filter( $the_content ) {
$new_content = str_replace( 'href',
'onClick="recordOutboundLink( this );return false;" href'
, $the_content );
return $new_content;
}
add_filter( 'the_content', 'newga_content_filter' );
function newga_content_filter( $the_content ) {
$new_content = str_replace( 'href',
'onClick="recordOutboundLink( this );return false;" href'
, $the_content );
return $new_content;
}
●
Add Javascript function call to any links found in content

31. Full filter hook implementation

32. Full filter hook implementation
add_action( 'wp_footer', 'footer_analytics_code' );
function footer_analytics_code() { ?>
<script type="text/javascript">
function recordOutboundLink( link ) {
ga( 'send', 'event', 'Outbound Links', 'Click',
link.href, {
'transport': 'beacon',
'hitCallback': function() {
document.location = link.href;
}
} );
}
</script>
<?php }
add_action( 'wp_footer', 'footer_analytics_code' );
function footer_analytics_code() { ?>
<script type="text/javascript">
function recordOutboundLink( link ) {
ga( 'send', 'event', 'Outbound Links', 'Click',
link.href, {
'transport': 'beacon',
'hitCallback': function() {
document.location = link.href;
}
} );
}
</script>
<?php }

33. Adding a shortcode
●
Simple codes used in a post or page to insert content
– [gallery]
– [gallery id="123" size="medium"]
●
Can be used to output special code before and afer content
– [style1]My text block[/style1]
– Ofen introduced by themes
●
If you repeatedly insert similar code on site, make a
shortcode

34. Shortcode Implementation
●
Since shortcodes are found anywhere within posts / pages,
they must return their output
add_shortcode( 'dq', 'mysite_div_quote' );
function mysite_div_quote( $atts, $content = null ) {
if ( !empty( $content ) ) {
return '<div class="site_quote" style="text-
align:right">' . $content . '</div>';
}
}
add_shortcode( 'dq', 'mysite_div_quote' );
function mysite_div_quote( $atts, $content = null ) {
if ( !empty( $content ) ) {
return '<div class="site_quote" style="text-
align:right">' . $content . '</div>';
}
}
[dq]This is my text[/dq][dq]This is my text[/dq]

35. Shortcode Implementation
add_shortcode( 'twitterfeed', 'twitter_embed_shortcode' );
function twitter_embed_shortcode( $args ) {
extract( shortcode_atts( array(
'user_name' => 'ylefebvre'
), $args ) );
if ( !empty( $user_name ) ) {
$output = '<a class="twitter-timeline" href="';
$output .= esc_url( 'https://twitter.com/' .
$user_name );
$output .= '">Tweets by ' . esc_html( $user_name );
$output .= '</a><script async ';
$output .= 'src="//platform.twitter.com/widgets.js"';
$output .= ' charset="utf-8"></script>';
} else {
$output = '';
}
return $output;
}
add_shortcode( 'twitterfeed', 'twitter_embed_shortcode' );
function twitter_embed_shortcode( $args ) {
extract( shortcode_atts( array(
'user_name' => 'ylefebvre'
), $args ) );
if ( !empty( $user_name ) ) {
$output = '<a class="twitter-timeline" href="';
$output .= esc_url( 'https://twitter.com/' .
$user_name );
$output .= '">Tweets by ' . esc_html( $user_name );
$output .= '</a><script async ';
$output .= 'src="//platform.twitter.com/widgets.js"';
$output .= ' charset="utf-8"></script>';
} else {
$output = '';
}
return $output;
}

36. Shortcode Implementation
●
The resulting shortcode [twitterfeed user_name='WordPress'][twitterfeed user_name='WordPress']
becomes

37. More tools
Admin menu

38. More tools
Admin menu Admin pages

39. More tools
Admin menu Admin pages Extend user editor

40. More tools
Admin menu Admin pages
Custom post types
Extend user editor

41. More tools
Admin menu Admin pages
Custom post types
Add custom meta
boxes to any editor
Extend user editor

42. More tools
Admin menu Admin pages
Custom post types
Add custom meta
boxes to any editor
Extend user editor
New
widgets

43. More tools
Admin menu Admin pages
Custom post types
Add custom meta
boxes to any editor
Extend user editor
New
widgets
Plugin translation

44. More tools
●
Store and retrieve plugin settings from site database
●
Query posts
●
Insert styles and scripts in page header and footer
●
Directly access database

45. Considerations
●
Function names must be diferent from WordPress core
functions and other plugins
●
Entire content is evaluated each time site
is rendered
●
A single error will usually bring down the
entire site
●
Using a local development environment is much safer than
developing on live site

46. Publishing your plugin
●
wordpress.org oficial repository
– Free plugins only
– Must follow GPL license
– Discoverable from admin plugins page
– Built-in update mechanism
●
Marketplaces (CodeCanyon, MOJO Marketplace, WPEden,
etc…)
– Paid premium plugins
– Need to implement custom update method
– Higher level of support expectation

47. Conclusions
●
Creating a plugin can be created using very few lines of code
●
When you deliver customer projects, make plugins over
adding code to functions.php
●
Consider distributing your plugins to WordPress community

48. Recommended Readings
●
WordPress Plugin Development
Cookbook, Second Edition by
Yannick Lefebvre, Packt Publishing
●
WordPress Codex
(codex.wordpress.com)
●
PHP.net
●
StackOverflow.com Programming
Samples
●
Today's presentation and code
samples available at:
– http://ylefebvre.ca/wcmtl2017 WPDCSEd50

49. Questions?
Thank you for attending this talk on
Plugin Development Demystified
Yannick Lefebvre
@ylefebvre
ylefebvre.ca