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Hooked on WordPress
An Introduction to Actions & Filters
@shawnhooper - shawnhooper.ca
WordCamp Ottawa
July 2019
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@shawnhooper - shawnhooper.ca
Hi, I’m Shawn
• Developing for the web since the mid-90s
• Using WordPress since ~ version 2.8 (2009)
• Director of IT at Actionable.co
• Lives in Ottawa, Canada
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What is a Hook ?
@shawnhooper - shawnhooper.ca
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What is a Hook ?
@shawnhooper - shawnhooper.ca
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What is a Hook ?
@shawnhooper - shawnhooper.ca
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What is a Hook ?
@shawnhooper - shawnhooper.ca
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What is a Hook ?
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What is a Hook ?
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What is a Hook ?
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Hooks allow you to change the core functionality
of WordPress.
They are implemented through
themes and plugins.
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What is a Hook ?
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Never Modify Core
( or Parent Themes )
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What is a Hook ?
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WordPress Plugin API
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What is a Hook ?
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There are two kinds of hooks in WordPress:
Actions and Filters
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Action Hooks
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Flickr: ifindkarma
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Action Hooks
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An action is triggered when a
specific event takes place in WordPress.
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Action Hooks
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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
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Action Hooks
@shawnhooper - shawnhooper.ca
do_action( $tag );
do_action ($tag, $arg_a, $arg_b, $arg_c );
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Action Hooks
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do_action ( ‘wp_head’ );
wp-includes\general-template.php
do_action ( ‘save_post’, $post->ID, $post, $update );
wp-includes\post.php
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Action Hooks
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add_action ( $hook, $function_to_add );
add_action( $hook, $function_to_add, $priority, $accepted_args );
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Action Hooks
@shawnhooper - shawnhooper.ca
The optional priority parameter allows you to
specify the order in which the hooked actions
will run.
The default value is 10.
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Action Hooks
@shawnhooper - shawnhooper.ca
add_action( ‘wp_head’, ‘smh_add_metadesc’);
function smh_add_metadesc() {
echo ‘’;
}
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Filter Hooks
@shawnhooper - shawnhooper.ca
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Filter Hooks
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Filters are functions that WordPress passes data
through, at certain points in execution, just before
taking some action with the data.
Source: WordPress Codex
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Filter Hooks
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$filter = apply_filters( $tag, $value );
$value = apply_filters ($tag, $value, $var1, $var2, … );
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Filter Hooks
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$content = apply_filters ( ‘the_content’, $content );
wp-includes\post-template.php
$atts = apply_filters( 'wp_mail', compact( 'to', 'subject',
'message', 'headers', 'attachments' ) );
wp-includes\pluggable.php
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Filter Hooks
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add_filter ( $tag, $function_to_add );
add_filter( $tag, $function_to_add, $priority, $accepted_args );
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Filter Hooks
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The optional priority parameter allows you to
specify the order in which the hooked filters
will run.
The default value is 10.
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Filter Hooks
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function smh_old_post_notice($content) {
$days = floor( ( time() - get_the_date( 'U' ) ) / ( 60 * 60 * 24 ) );
if ( $days > 365 ) {
$content = '
This is an older post. Beware of
possible out-of-date advice.
' . $content;
}
return $content;
}
add_filter(‘the_content’, ‘smh_old_post_notice’);
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Hooks in Themes
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99% of the time, you’ll use hooks and filters
in plugins.
But there are some times where it’s useful to put these
functions into your theme’s functions.php file.
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Hooks in Themes
@shawnhooper - shawnhooper.ca
Examples:
Theme Activation Hook
Modifying the “Read More” Text
Filtering Menu Content
Setting up the Customizer
These are all things that are isolated to the active theme. If you
change themes, your site won’t break.
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That’s the basics….
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Extending Plugins
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WordPress Core isn’t the only thing that can be extended
with hooks.
A well written plugin can also expose Actions and Filters
that will allow you to add, remove, or modify
its functionality.
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Variable Hooks
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Some hooks include variables in their names. These are very
powerful hooks that allow you to interact with only specific objects
in WordPress.
save_post_{$post_type}
ex: add_action ( ‘save_post_page’, ‘my_function’);
would only trigger when a page is being saved, not a post.
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Pluggable Functions
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A set of core WordPress functions that can be overridden in your
plugins.
They can all be found in wp-includes\pluggable.php
NOTE: No new pluggable functions are being added. They are
being replaced with filters, a more flexible solution.
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Pluggable Functions
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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
}
}
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Removing Hooks
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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 );
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Removing Hooks
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You can also remove ALL hooks that have already been put into
place.
remove_all_actions( $tag, $priority );
remove_all_filters ($tag, $priority );
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Naming Conventions
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Function names must be unique
(or you risk the White Screen of Death!)
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Naming Conventions
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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’) );
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@shawnhooper - shawnhooper.ca
Thank You!
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@shawnhooper - shawnhooper.ca
Questions ?
Twitter: @shawnhooper
WordPress Slack: @shooper
Slides & Notes Posted on:
www.shawnhooper.ca
E-Mail:
[email protected]