Adds a callback function to a filter hook.

Description

WordPress offers filter hooks to allow plugins to modify various types of internal data at runtime.

A plugin can modify data by binding a callback to a filter hook. When the filter is later applied, each bound callback is run in order of priority, and given the opportunity to modify a value by returning a new value.

The following example shows how a callback function is bound to a filter hook.

Note that $example is passed to the callback, (maybe) modified, then returned:

function example_callback( $example ) {
    // Maybe modify $example in some way.
    return $example;
}
add_filter( 'example_filter', 'example_callback' );

Bound callbacks can accept from none to the total number of arguments passed as parameters in the corresponding apply_filters() call.

In other words, if an apply_filters() call passes four total arguments, callbacks bound to it can accept none (the same as 1) of the arguments or up to four. The important part is that the $accepted_args value must reflect the number of arguments the bound callback actually opted to accept. If no arguments were accepted by the callback that is considered to be the same as accepting 1 argument. For example:

// Filter call.
$value = apply_filters( 'hook', $value, $arg2, $arg3 );

// Accepting zero/one arguments.
function example_callback() {
    ...
    return 'some value';
}
add_filter( 'hook', 'example_callback' ); // Where $priority is default 10, $accepted_args is default 1.

// Accepting two arguments (three possible).
function example_callback( $value, $arg2 ) {
    ...
    return $maybe_modified_value;
}
add_filter( 'hook', 'example_callback', 10, 2 ); // Where $priority is 10, $accepted_args is 2.

_Note:_ The function will return true whether or not the callback is valid.
It is up to you to take care. This is done for optimization purposes, so everything is as quick as possible.

Parameters

$hook_namestringrequired

The name of the filter to add the callback to.

$callbackcallablerequired

The callback to be run when the filter is applied.

$priorityintoptional

Used to specify the order in which the functions associated with a particular filter are executed.
Lower numbers correspond with earlier execution, and functions with the same priority are executed in the order in which they were added to the filter.

Default:10

$accepted_argsintoptional

The number of arguments the function accepts.

Default:1

Return

More Information

• Hooked functions can take extra arguments that are set when the matching do_action() or apply_filters() call is run. For example, the comment_id_not_found action will pass the comment ID to each callback.
• Although you can pass the number of $accepted_args, you can only manipulate the $value. The other arguments are only to provide context, and their values cannot be changed by the filter function.
• You can also pass a class method as a callback.

Static class method:

add_filter( 'media_upload_newtab', array( 'My_Class', 'media_upload_callback' ) );

Instance method:

add_filter( 'media_upload_newtab', array( $this, 'media_upload_callback' ) );

• You can also pass an an anonymous function as a callback. For example:

add_filter( 'the_title', function( $title ) { return '<strong>' . $title . '</strong>'; } );

Source

function add_filter( $hook_name, $callback, $priority = 10, $accepted_args = 1 ) {
	global $wp_filter;

	if ( ! isset( $wp_filter[ $hook_name ] ) ) {
		$wp_filter[ $hook_name ] = new WP_Hook();
	}

	$wp_filter[ $hook_name ]->add_filter( $hook_name, $callback, $priority, $accepted_args );

	return true;
}

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