WordPress is a popular Content Management System (CMS) that provides various hooks to modify or extend its core functionality. Among those hooks, the add_filter and apply_filters functions are two of the most commonly used ones. These functions are used to modify the value of a variable or a function result using a callback function. In this article, we will discuss the differences between add_filter and apply_filters.
WordPress Filters
Before we delve into the differences between add_filter and apply_filters, let’s first understand what WordPress filters are.
In WordPress, filters are hooks that allow you to modify the value of a variable or a function result before it is displayed on the front-end or saved in the database. Filters are defined by WordPress core or by third-party plugins and themes.
WordPress provides two types of filters: Actions and Filters. Actions are used to execute a block of code at a specific point in WordPress execution, while Filters are used to modify a value before it is returned or saved.
add_filter
The add_filter function is used to register a filter hook. It accepts three parameters:
add_filter( $tag, $function_to_add, $priority, $accepted_args );$tag: The name of the filter hook to register.$function_to_add: The callback function that will be executed when the filter hook is applied.$priority: (optional) The priority of the filter hook. The default priority is 10.$accepted_args: (optional) The number of arguments the callback function accepts. The default value is 1.
Here’s an example of how to use add_filter:
function wpd_filter_function( $content ) {
return str_replace( 'foo', 'bar', $content );
}
add_filter( 'the_content', 'wpd_filter_function' );In this example, we registered a filter hook called the_content using add_filter. We then defined a callback function called wpd_filter_function that replaces all instances of the string ‘foo’ with ‘bar’ in the content. Finally, we added our callback function to the the_content filter hook using add_filter.
apply_filters
The apply_filters function is used to execute a filter hook. It accepts two parameters:
apply_filters( $tag, $value );$tag: The name of the filter hook to execute.$value: The value that will be modified by the filter hook.
Here’s an example of how to use apply_filters:
$title = 'Hello World';
$title = apply_filters( 'wpd_filter_hook', $title );In this example, we executed a filter hook called wpd_filter_hook using apply_filters. We passed a string ‘Hello World’ as the value to be modified by the filter hook. The filter hook may modify the value of $title.
Differences between add_filter and apply_filters
The main difference between add_filter and apply_filters is that add_filter is used to register a filter hook, while apply_filters is used to execute a filter hook.
When we register a filter hook using add_filter, we define the callback function that will modify the value. We can also specify the priority and the number of arguments the callback function accepts.
When we execute a filter hook using apply_filters, we pass the value that will be modified by the filter hook. We also pass the name of the filter hook that will be executed.
In summary, add_filter is used to register a filter hook, while apply_filters is used to execute a filter hook. add_filter allows us to define the callback function, priority