Title: Filters Author: Samuel Sidler Published: September 16, 2014 Last modified: November 17, 2022 --- # Filters ## In this article * [Add Filter](https://developer.wordpress.org/plugins/hooks/filters/?device=c&output_format=md#add-filter) - [Additional Parameters](https://developer.wordpress.org/plugins/hooks/filters/?device=c&output_format=md#additional-parameters) - [Example](https://developer.wordpress.org/plugins/hooks/filters/?device=c&output_format=md#example) [ Back to top](https://developer.wordpress.org/plugins/hooks/filters/?device=c&output_format=md#wp--skip-link--target) **Filters** are one of the two types of [Hooks](https://developer.wordpress.org/plugins/hooks/). They provide a way for functions to modify data during the execution of WordPress Core, plugins, and themes. They are the counterpart to [Actions](https://developer.wordpress.org/plugins/hooks/actions/). Unlike [Actions](https://developer.wordpress.org/plugins/hooks/actions/), filters are meant to work in an isolated manner, and should never have [side effects](https://en.wikipedia.org/wiki/Side_effect_(computer_science)) such as affecting global variables and output. Filters expect to have something returned back to them. ## 󠀁[Add Filter](https://developer.wordpress.org/plugins/hooks/filters/?device=c&output_format=md#add-filter)󠁿 The process of adding a filter includes two steps. First, you need to create a Callback function which will be called when the filter is run. Second, you need to add your Callback function to a hook which will perform the calling of the function. You will use the [add_filter()](https://developer.wordpress.org/reference/functions/add_filter/) function, passing at least two parameters: 1. `string $hook_name` which is the name of the filter you’re hooking to, and 2. `callable $callback` the name of your callback function. The example below will run when the `the_title` filter is executed. ```php function wporg_filter_title( $title ) { return 'The ' . $title . ' was filtered'; } add_filter( 'the_title', 'wporg_filter_title' ); ``` Lets say we have a post title, “Learning WordPress”, the above example will modify it to be “The Learning WordPress was filtered”. You can refer to the [Hooks](https://developer.wordpress.org/plugins/hooks/) chapter for a list of available hooks. As you gain more experience, looking through WordPress Core source code will allow you to find the most appropriate hook. ### 󠀁[Additional Parameters](https://developer.wordpress.org/plugins/hooks/filters/?device=c&output_format=md#additional-parameters)󠁿 [add_filter()](https://developer.wordpress.org/reference/functions/add_filter/) can accept two additional parameters, `int $priority` for the priority given to the callback function, and `int $accepted_args` for the number of arguments that will be passed to the callback function. For detailed explanation of these parameters please read the article on [Actions](https://developer.wordpress.org/plugins/hooks/actions/). ### 󠀁[Example](https://developer.wordpress.org/plugins/hooks/filters/?device=c&output_format=md#example)󠁿 To add a CSS class to the `` tag when a certain condition is met: ```php function wporg_css_body_class( $classes ) { if ( ! is_admin() ) { $classes[] = 'wporg-is-awesome'; } return $classes; } add_filter( 'body_class', 'wporg_css_body_class' ); ``` First published September 16, 2014 Last updated November 17, 2022 [ Previous: Actions](https://developer.wordpress.org/plugins/hooks/actions/) [ Next: Custom Hooks](https://developer.wordpress.org/plugins/hooks/custom-hooks/)