What Is a WordPress Filter? The Ultimate Beginner‘s Guide for 2023
If you‘re new to WordPress development, you may have heard the term "filter" thrown around without fully understanding what it means. Filters are one of the key building blocks that make WordPress so flexible and extendable. Essentially, filters allow developers to modify or enhance the default functionality of WordPress by intercepting, processing and returning data at specific points in the code.
While this might sound complex, the basic concept of filters is actually quite simple. In this beginner‘s guide, we‘ll break down everything you need to know about WordPress filters, how they work, and how you can start using them on your own site. Let‘s dive in!
What is a WordPress Filter?
In simple terms, a WordPress filter is a function that takes in data, modifies it in some way, and then returns the modified data back to WordPress. Filters "hook into" specific points in the WordPress code base, allowing developers to change how WordPress behaves without editing core files directly.
For example, let‘s say you wanted to automatically append a "Read More" link to the end of your post excerpts. Rather than manually typing this for every post, you could use a filter to intercept the post excerpt, add the link, and return the modified excerpt to be displayed.
Filters are incredibly powerful because they allow you to change almost any text-based aspect of your site – from content and excerpts, to emails, URLs, queries, and more. Theme and plugin developers make heavy use of filters, but even non-coders can utilize them through plugins or by adding code snippets.
How Do WordPress Filters Work?
Under the hood, filters are just PHP functions that are "hooked" to specific WordPress events using the add_filter() function. When the event occurs, WordPress executes the hooked function, passes in any relevant data, then receives back the function‘s return value.
The basic syntax for adding a filter looks like this:
add_filter( ‘hook_name‘, ‘your_filter_function‘ );
function your_filter_function( $data ) {
// Modify $data in some way
return $modified_data;
}
The first parameter is the name of the hook you want to attach to, and the second parameter is the name of your custom filter function.
Your filter function takes in the data from the hook as a parameter (or sometimes multiple parameters), performs some operation on that data, then returns the modified data back to WordPress. This modified data is then used in place of the original.
It‘s important to note that filters must always return a value, even if they didn‘t modify the incoming data. Failing to return will result in your filtered data being empty.
Removing Filters and Checking Filter Existence
In addition to the add_filter() function for attaching filters, WordPress also provides functions for removing filters and checking if filters are attached to a hook:
remove_filter(): Removes a previously added filter from a hook.
has_filter(): Checks if any filters are attached to a given hook name.
doing_filter(): Retrieves the name of the filter currently being processed.
These functions give you more control over when and how your filters are applied.
The Difference Between Filters, Actions, and Hooks
If you‘re learning about WordPress development, you‘ll often hear the terms actions and hooks mentioned alongside filters. While related, these terms each have a distinct meaning:
Hooks are generic spots in the WordPress codebase where you can insert your own code. They come in two flavors – action hooks and filter hooks.
Actions allow you to do something at a specific point, like adding extra content or triggering an event. However, unlike filters, actions don‘t get passed in data and don‘t return anything.
Filters allow you to modify data at certain points before it‘s stored or output to the browser. They take in data, let you change it, then pass it back for WordPress to continue using.
So in summary – hooks are the overall system, while actions and filters are the two types of hooks available. Filters deal with data modification, while actions are used for injecting functionality.
Practical Use Cases and Examples
So what can you actually do with filters? The possibilities are nearly endless, but here are a few common examples:
Modify the length or content of excerpts
Customize the default sort order of queries
Add custom fields or metadata to posts
Modify the output of shortcodes
Change the "Read More" link text
Customize dashboard widgets
Modify plugin settings or behavior
One practical example would be changing your excerpt length. By default, WordPress excerpts are 55 words long. But you can use the excerpt_length filter to change this:
function custom_excerpt_length( $length ) {
return 30; // Set excerpt length to 30 words
}
add_filter( ‘excerpt_length‘, ‘custom_excerpt_length‘ );
This intercepts the default excerpt length, changes it to 30 words, then returns the new length back to WordPress to use.
Another useful example is modifying the default post query. Let‘s say you wanted to exclude a certain category from your blog index. You could use the pre_get_posts filter like so:
function exclude_category( $query ) {
if ( $query->is_home() && $query->is_main_query() ) {
$query->set( ‘cat‘, ‘-5‘ );
}
return $query;
}
add_filter( ‘pre_get_posts‘, ‘exclude_category‘ );
This checks if we‘re on the blog home page and if it‘s the main query, then modifies the query to exclude category ID 5 before returning the modified query object.
These are just a couple basic examples, but they demonstrate the power and flexibility of filters. You can find a huge variety of practical uses in our WordPress Tutorials archive.
How to Use Filters as a Non-Developer
If you‘re not comfortable with code, don‘t worry – you can still take advantage of filters on your WordPress site.
Many WordPress plugins provide settings or options that allow you to customize your site‘s behavior without writing any code. Under the hood, these plugins are often using filters to modify WordPress‘ default functionality based on your settings. So in many cases, you can "set it and forget it" and let the plugin handle the filtering for you.
However, sometimes you may want to make a change that requires a bit of custom code. In these cases, you have a few options:
Use a "code snippets" plugin that lets you safely add custom code. These plugins create a separate place for your snippets so you don‘t have to modify your theme files directly.
Create a child theme and add your filter code there. This lets you modify your theme without losing those changes when the parent theme is updated.
Hire a developer to implement the filters you need in a custom plugin or child theme.
The key is to never add custom filter code directly to your theme or plugin files. If the theme or plugin is updated, your changes will be overwritten. Always use a child theme, custom plugin, or snippets plugin.
It‘s also crucial to properly test any filters before deploying to a live site, and always keep regular backups in case something goes wrong. If you‘re not confident in your coding abilities, it‘s best to leave filter customizations to an experienced developer.
Filters in 2023 and Beyond
As WordPress evolves, so too does the best practices around using filters. In the past, it was common to jam a bunch of filters and custom code into your theme‘s functions.php file. But these days, developers recommend a more modular approach.
Instead of dumping everything into your theme, it‘s better to create separate child themes or custom plugins for your filter code. This makes your customizations more portable and less prone to breakage from theme or WordPress core updates.
There are also more tools available than ever for working with filters and custom code. In addition to countless "code snippet" plugins, platforms like GitHub make it easy for developers to share, collaborate on, and version control their filter customizations.
Looking forward, it‘s likely that filters will continue to play a huge role in WordPress development. The new Gutenberg block editor introduced a number of new filter hooks for customizing the editor experience, and this trend will likely continue as WordPress moves toward a more "block-based" future.
Keep Learning with WPBeginner
We hope this article helped demystify WordPress filters and showed you how powerful they can be. Whether you‘re a budding developer looking to extend WordPress, or a site owner wanting to customize some aspect of your site, understanding how filters work is key.
For more beginner-friendly WordPress tutorials, be sure to check out our full archive of training material. And if you want to stay on top of the latest WordPress tips, tricks, and best practices, subscribe to the WPBeginner YouTube channel or follow us on Facebook and Twitter.
Do you have any other questions about WordPress filters? Let us know in the comments and we‘ll do our best to help out!
