WordPress Hooks: Actions and Filters

Hooks are the most important idea in WordPress development. A hook is a place in the WordPress code where you can run your own function, or change some data, without editing the core files. Themes and plugins both use hooks. There are two types: actions and filters.

Point	Action	Filter
Purpose	Do something at a moment	Change a value and give it back
Add with	add_action()	add_filter()
Run by	do_action()	apply_filters()
Return value	Not needed	Must return the (changed) value

Actions

An action runs your function when WordPress reaches that moment. Here the function is attached to a hook which we create ourselves with do_action():

function tc_say_hello( $name ) {
	echo "Hello, $name!\n";
}
add_action( 'tc_greeting', 'tc_say_hello' );

do_action( 'tc_greeting', 'Riya' );
echo 'Times the action ran: ' . did_action( 'tc_greeting' );

Filters

A filter receives a value, changes it and returns it. Always return the value, even if you did not change it, otherwise the content will disappear.

function tc_add_signature( $content ) {
	return $content . ' -- Team Tutorials Class';
}
add_filter( 'tc_message', 'tc_add_signature' );

echo apply_filters( 'tc_message', 'Thanks for reading' );

Priority and accepted arguments

add_action( $hook, $callback, $priority = 10, $accepted_args = 1 ). A lower priority number runs earlier. The default is 10.

add_action( 'tc_steps', function () { echo "B (priority 10)\n"; } );
add_action( 'tc_steps', function () { echo "A (priority 5)\n"; }, 5 );
add_action( 'tc_steps', function () { echo "C (priority 20)\n"; }, 20 );
do_action( 'tc_steps' );

If the hook passes more than one value, say how many your function wants as the fourth parameter:

add_filter( 'tc_price', function ( $price, $tax_percent ) {
	return $price + ( $price * $tax_percent / 100 );
}, 10, 2 );

echo apply_filters( 'tc_price', 200, 18 );

Remove a hook

function tc_noisy() { echo "noisy\n"; }
add_action( 'tc_event', 'tc_noisy' );
remove_action( 'tc_event', 'tc_noisy' );
do_action( 'tc_event' );
echo has_action( 'tc_event', 'tc_noisy' ) ? 'still attached' : 'removed';

Real WordPress hooks

Hook	Type	When it runs
init	action	After WordPress is loaded. Register post types and taxonomies here
after_setup_theme	action	Theme setup, add_theme_support
wp_enqueue_scripts	action	Load CSS and JS on the front
wp_head, wp_footer	action	Inside the head and before the closing body tag
widgets_init	action	Register sidebars and widgets
save_post	action	After a post is saved
admin_menu	action	Add pages to the dashboard menu
the_content	filter	Change the content of a post before it is shown
the_title	filter	Change post titles
body_class	filter	Add classes to the body tag

A real filter on the_content

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