Hook a function or method to a specific filter action.

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.

Signature

add_filter( $tag, $function_to_add, $priority = 10, $accepted_args = 1 )
tag
 (string) The name of the filter to hook the $function_to_add callback to.
function_to_add
 (callable) The callback to be run when the filter is applied.
priority
 (int) Optional. Used to specify the order in which the functions associated with a particular action are executed. Default 10. Lower numbers correspond with earlier execution, and functions with the same priority are executed in the order in which they were added to the action.
Default: 10
accepted_args
 (int) Optional. The number of arguments the function accepts. Default 1.
Default: 1

Return

(true)

Source

function add_filter( $tag, $function_to_add, $priority = 10, $accepted_args = 1 ) {
	global $wp_filter;
	if ( ! isset( $wp_filter[ $tag ] ) ) {
		$wp_filter[ $tag ] = new WP_Hook();
	}
	$wp_filter[ $tag ]->add_filter( $tag, $function_to_add, $priority, $accepted_args );
	return true;
}
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