Beginner WordPress Developer

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WordPress hooks

Filter Hooks

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There are two types of WordPress hooks, actions, and filters.

In this lesson, we’ll focus on filters, but check out the actions lesson for more information on action hooks.

What are filter hooks?

Filters allow you to modify, or filter, some data at a specific point, which will be used later on.

In order to make use of a filter, you register a function in your code to a pre-existing filter hook, which is known as a callback function.

To better explain this, let’s look at a filter called the_content.

This filter is defined in the wp-includes/post-template.php file, inside the function which is used in theme templates whenever the template needs to render any post or page content.

Inside that function, we see this code

$content = apply_filters( 'the_content', $content );

Here the apply_filters function defines the filter hook, with the hook name of the_content.

You will notice that a $content variable is passed as an argument of apply_filters and that the value of apply_filters is assigned back to a variable, in this case, again, the $content variable.

If we look a little higher in this function, the $content variable is assigned the value of the get_the_content function, which is a WordPress core function that retrieves the value of the post_content field for the current post or page in the posts table.

So the apply_filters function registers the filter hook, passes the value of $content at this point in the code execution to any callback functions registered on this hook, and requires the updated value to be passed back.

Using filter hooks

To register your callback function on a filter you use the WordPress add_filter function.

You will need to pass the hook name and the name of your callback function as arguments to the add_filter function.

Let’s take a look at what this looks like in a theme’s functions.php file.

In your code editor, navigate to your currently active theme’s functions.php file, and open it.

If your theme doesn’t have a functions.php file, you can create one in the root of your theme directory. Just make sure it’s named functions.php, and has the opening PHP tag at the top of the file.

Then, add the following code to your functions.php file to hook a callback function into the the_content filter hook.

add_filter( 'the_content', 'wp_learn_amend_content' );

Then, create the callback function, using the PHP function syntax, which accepts the relevant argument from the filter.

function wp_learn_amend_content( $content ) {
// do some things that update $content
return $content;

You don’t have to name the argument the same as the variable name passed from the filter, but it does make it easier if you do.

Notice that you have to return the updated data. This is so that the original variable being updated from the apply_filters call gets the updated data.

For example, let’s say you wanted to add something to the end of the content of each post, you could append it to the $content variable like this.

add_filter( 'the_content', 'wp_learn_amend_content' );
function wp_learn_amend_content( $content ) {
$additional_content = '<!-- wp:paragraph --><p>Filtered through <i>the_content</i></p><!-- /wp:paragraph -->';
$content = $content . $additional_content;

return $content;

In this example, you’re adding some text in a paragraph block, which renders at the bottom of each post on the front end.

It’s very important to always return something back from a filter callback, ideally, the modified content of the variable passed to the filter. Not returning something will cause a fatal error on your WordPress site.

Let’s take a look at what that looks like in our WordPress site.

If you view any post or page on the front end, you will see the text “Filtered through the_content” at the bottom of the content.

So, as you can see from this example, filter hooks allow you to modify certain pieces of data at a specific point in the code execution.