Slides from talk given at WordCamp Columbus 2015.

1. Hooked on WordPress
An Introduction to Actions & Filters
@shawnhooper - shawnhooper.ca
WordCamp Columbus
July 17, 2015

2. @shawnhooper - shawnhooper.ca
Hi, I’m Shawn
• Developing for the web since the mid-90s
!
• Using WordPress since ~ version 2.8 (2009)
!
• Chief Technology Officer at Actionable Books
!
• Lives in Ottawa, Canada

3. What is a Hook ?
@shawnhooper - shawnhooper.ca

4. What is a Hook ?
@shawnhooper - shawnhooper.ca

5. What is a Hook ?
@shawnhooper - shawnhooper.ca

6. What is a Hook ?
@shawnhooper - shawnhooper.ca

7. What is a Hook ?
@shawnhooper - shawnhooper.ca

8. What is a Hook ?
@shawnhooper - shawnhooper.ca

9. What is a Hook ?
@shawnhooper - shawnhooper.ca
Never Modify Core

10. What is a Hook ?
@shawnhooper - shawnhooper.ca
Hooks allow you to change the core functionality
of WordPress.
!
They are implemented through
themes and plugins.

11. What is a Hook ?
@shawnhooper - shawnhooper.ca
WordPress Plugin API

12. What is a Hook ?
@shawnhooper - shawnhooper.ca
There are two kinds of hooks in WordPress:
!
Actions and Filters

13. Action Hooks
@shawnhooper - shawnhooper.ca
Flickr: ifindkarma

14. Action Hooks
@shawnhooper - shawnhooper.ca
An action is triggered when a
specific event takes place in WordPress.

15. Action Hooks
@shawnhooper - shawnhooper.ca
What can you do in an action?!
!
Modify data in the database
Send an e-mail
Interact with APIs
Modify code being sent to the browser

16. Action Hooks
@shawnhooper - shawnhooper.ca
do_action( $tag );
!
do_action ($tag, $arg_a, $arg_b, $arg_c );

17. Action Hooks
@shawnhooper - shawnhooper.ca
do_action ( ‘wp_head’ );!
!
wp-includesgeneral-template.php
do_action ( ‘save_post’, $post->ID, $post, $update );!
!
wp-includespost.php

18. Action Hooks
@shawnhooper - shawnhooper.ca
add_action ( $hook, $function_to_add );
add_action( $hook, $function_to_add, $priority, $accepted_args );

19. Action Hooks
@shawnhooper - shawnhooper.ca
The optional priority parameter allows you to
specify the order in which the hooked actions
will run.
!
The default value is 10.

20. Action Hooks
@shawnhooper - shawnhooper.ca
add_action( ‘wp_head’, ‘smh_add_metadesc’);
function smh_add_metadesc() {
echo ‘<meta name=“description” value=“Hello!” />’;
}

21. Filter Hooks
@shawnhooper - shawnhooper.ca

22. Filter Hooks
@shawnhooper - shawnhooper.ca
Filters are functions that WordPress passes data
through, at certain points in execution, just before
taking some action with the data.
!
Source: WordPress Codex

23. Filter Hooks
@shawnhooper - shawnhooper.ca
$filter = apply_filters( $tag, $value );
!
$value = apply_filters ($tag, $value, $var1, $var2, … );

24. Filter Hooks
@shawnhooper - shawnhooper.ca
$content = apply_filters ( ‘the_content’, $content );!
!
wp-includespost-template.php
$atts = apply_filters( 'wp_mail', compact( 'to', 'subject',
'message', 'headers', 'attachments' ) );
wp-includespluggable.php

25. Filter Hooks
@shawnhooper - shawnhooper.ca
add_filter ( $tag, $function_to_add );
add_filter( $tag, $function_to_add, $priority, $accepted_args );

26. Filter Hooks
@shawnhooper - shawnhooper.ca
The optional priority parameter allows you to
specify the order in which the hooked filters
will run.
!
The default value is 10.

27. Filter Hooks
@shawnhooper - shawnhooper.ca
function smh_old_post_notice($content) {
!
$days = floor( ( time() - get_the_date( 'U' ) ) / ( 60 * 60 * 24 ) );
!
if ( $days > 365 ) {
$content = '<div class="old_post">This is an older post. Beware of
possible out-of-date advice.</div>' . $content;
}
!
return $content;
}
!
add_filter(‘the_content’, ‘smh_old_post_notice’);

28. Hooks in Themes
@shawnhooper - shawnhooper.ca
99% of the time, you’ll use hooks and filters
in plugins.
!
But there are some times where it’s useful to put these
functions into your theme’s functions.php file.

29. Hooks in Themes
@shawnhooper - shawnhooper.ca
Examples:
!
Theme Activation Hook
Modifying the “Read More” Text
Filtering Menu Content
Setting up the Customizer
!
These are all things that are isolated to the active theme. If you
change themes, your site won’t break.

30. That’s the basics….
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31. Extending Plugins
@shawnhooper - shawnhooper.ca
WordPress Core isn’t the only thing that can be extended
with hooks.
!
A well written plugin can also expose Actions and Filters
that will allow you to add, remove, or modify
its functionality.

32. Variable Hooks
@shawnhooper - shawnhooper.ca
Some hooks include variables in their names. These are very
powerful hooks that allow you to interact with only specific objects
in WordPress.
!
save_post_{$post_type}
ex: add_action ( ‘save_post_page’, ‘my_function’);
!
would only trigger when a page is being saved, not a post.

33. Pluggable Functions
@shawnhooper - shawnhooper.ca
A set of core WordPress functions that can be overridden in your
plugins.
!
They can all be found in wp-includespluggable.php
!
!
NOTE: No new pluggable functions are being added. They are
being replaced with filters, a more flexible solution.

34. Pluggable Functions
@shawnhooper - shawnhooper.ca
Only one plugin can make use of these functions. For safety, wrap
the function with the function_exists() check:
if ( ! function_exists( ‘wp_mail’ ) ) {
function wp_mail($to, $subject, $message, $headers = ‘’) {
// do stuff
}
}

35. Removing Hooks
@shawnhooper - shawnhooper.ca
You can also remove hooks that have already been put into place.
!
!
remove_action( $tag, $function_name, $priority );
!
remove_filter ($tag, $function_to_remove, $priority );

36. Removing Hooks
@shawnhooper - shawnhooper.ca
You can also remove ALL hooks that have already been put into
place.
!
!
remove_all_actions( $tag, $priority );
!
remove_all_filters ($tag, $priority );

37. Naming Conventions
@shawnhooper - shawnhooper.ca
Function names must be unique
!
(or you risk the White Screen of Death!)

38. Naming Conventions
@shawnhooper - shawnhooper.ca
PRO TIP: Put your plugin in a class.!
!
add_action( ‘save_post’, array( $this, ‘send_email’) );
add_filter( ‘the_content’, array( $this, ‘old_post_warning’) );

39. @shawnhooper - shawnhooper.ca
Thank You!

40. @shawnhooper - shawnhooper.ca
Questions ?
Twitter: @shawnhooper
WordPress Slack: @shooper
Slides & Notes Posted on:
www.shawnhooper.ca
E-Mail:
shawn@actionablebooks.com