Introduction to WordPress Hooks

WordPress actions and filters, also known as hooks, are ways to “subscribe” to various parts of the WordPress codebase.

Actions allow you to execute your own code at specific points in the WordPress action hierarchy. As WordPress loads from top to bottom, various actions are triggered, which can be hooked into in order to execute code.

Filters, on the other hand, give you the ability to modify some data using your own code.

Filters are an easier concept to understand than actions, so we’ll start with that first.

Applying filters

Let’s look at a code sample:

$price = 3.99;
$price = apply_filters( 'get_price', $price );Code language: PHP (php)

The price starts at 3.99. Then apply_filters is called. It passes the variable $price to any subscribers to the filter.

Since there are no subscribers yet, the price will remain the same.That data flows through the filter and is returned as-is.

Now say, in another file, that I want to add some sales tax to this price. I can add myself as a subscriber to the price via the filter name.

add_filter( 'get_price', 'my_plugin_add_sales_tax_to_price' );
function my_plugin_add_sales_tax_to_price( $price ) {
    return $price + 1; // Add a dollar sales tax.
}Code language: PHP (php)

Now the $price variable is 4.99 after the filter has been applied.

Here’s a quick diagram of the flow between both plugins when apply_filters is called.

Let’s say a third plugin got in the mix, but this time at a lower priority. This one also subscribes to the same filter and subtracts 50 cents from the overall total.

add_filter( 'get_price', 'my_plugin_add_discount', 15 );
function my_plugin_add_discount( $price ) {
    return $price - 0.5; // remove fifty cents
}Code language: PHP (php)

Since this third plugin’s filter is a priority 15, it runs after the 2nd plugin’s filter.

The 2nd plugin didn’t specify a priority, so we’ll assume the priority is 10. When $price finally hits the new filter, the price is already set at 4.99. We just take that value, subtract 50 cents, and return it back.

Here’s a diagram showing the process:

One thing to keep in mind is that filters return something. Whether it’s the original value or not is up to those subscribing to the filter.

Let’s move onto actions, where we can execute code at certain predefined places.

Doing actions

Let’s look at another code sample, this time executing an action:

<ul>
<?php
    do_action( 'list_item' );
?>
</ul>Code language: PHP (php)

It doesn’t look like much, but the code is allowing others to add items to its list.

There’s an unordered list is currently empty. That is, until another item subscribes to the action. In this case, the action is named list_item.

Let’s add a action subscriber, which will output a list item.

add_action( 'list_item', 'my_plugin_2_add_list_item' );
function my_plugin_2_add_list_item() {
    ?>
    <li>List Item One</li>
    <?php
}Code language: PHP (php)

We now have “hooked in” to the action, and the result is an unordered list with one item:

<ul>
    <li>List Item One</li>
</ul>Code language: HTML, XML (xml)

What if another plugin, at priority 15, decides to also add a list item?

add_action( 'list_item', 'my_plugin_3_add_list_item', 15 );
function my_plugin_3_add_list_item() {
    ?>
    <li>List Item Two</li>
    <?php
}Code language: PHP (php)

You would end up with two list items:

<ul>
    <li>List Item One</li>
    <li>List Item Two</li>
</ul>Code language: HTML, XML (xml)

Let’s look at a diagram of the process:

The first plugin loads, and down the road performs a do_action, triggering the action. Two other plugins are hooked into (subscribed) to the action, and fire at different priorities.

To put it another way, actions are more like ripples in a pond. The initial action is tossing a rock in a pond. The ripples are all the actions that are based off of the impact.

Quiz Time!

In this introduction to hooks, I presented two new concepts: actions and filters. Let’s do a few brain teasers before moving on to the next lesson.

<?php
$price = 23;
$price = apply_filters( 'product_price', $price );
echo $price;Code language: PHP (php)

<?php
$price = 23;
$price = apply_filters( 'product_price', $price );

add_filter( 'product_price', 'my_plugin_new_price' );
function my_plugin_new_price( $price ) {
	return 24;
}
echo $price;Code language: PHP (php)

<?php
add_filter( 'product_price', 'my_plugin_new_price' );
function my_plugin_new_price( $price ) {
	return $price + 2;
}
add_filter( 'product_price', 'my_plugin_2_new_price', 9 );
function my_plugin_2_new_price( $price ) {
	return $price / 2;
}
$price = apply_filters( 'product_price', 20 );
echo $price;
Code language: PHP (php)

<?php
add_action( 'my_plugin_loaded', 'my_callback_plugin_loaded' );
function my_callback_plugin_loaded() {
	echo 'Plugin loaded';
}

do_action( 'my_plugin_loaded' );Code language: PHP (php)

<?php
add_action( 'my_plugin_loaded', 'my_callback_plugin_loaded' );
function my_callback_plugin_loaded() {
	echo apply_filters( 'my_sanitization_filter', 'Hello World!' );
}

add_filter( 'my_sanitization_filter', '__return_empty_string' );

do_action( 'my_plugin_loaded' );
Code language: PHP (php)