add_filter( string $tag, callable $function_to_add, int $priority = 10, int $accepted_args = 1 )

Hook a function or method to a specific filter action.


Description 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 Parameters

$tag

(string) (Required) The name of the filter to hook the $function_to_add callback to.

$function_to_add

(callable) (Required) 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. 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 value: 10

$accepted_args

(int) (Optional) The number of arguments the function accepts.

Default value: 1


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Return Return

(true)


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Source Source

File: wp-includes/plugin.php

function add_filter( $tag, $function_to_add, $priority = 10, $accepted_args = 1 ) {
	global $wp_filter, $merged_filters;

	$idx = _wp_filter_build_unique_id($tag, $function_to_add, $priority);
	$wp_filter[$tag][$priority][$idx] = array('function' => $function_to_add, 'accepted_args' => $accepted_args);
	unset( $merged_filters[ $tag ] );
	return true;
}


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Changelog Changelog

Changelog
Version Description
0.71 Introduced.

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More Information More Information

  • 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>'; } );
	


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User Contributed Notes User Contributed Notes

  1. Skip to note content
    Contributed by Codex

    Example
    The filter img_caption_shortcode is applied in media.php using the following call:

    // Allow plugins/themes to override the default caption template.
    $output = apply_filters( 'img_caption_shortcode', '', $attr, $content );
    if ( $output != '' )
    	return $output;
    

    The target filter function will be called with three arguments:

    ” <= This is normally the value the filter will be modifying
    $attr
    $content

    In order for the filter function to actually receive the full argument list, the call to add_filter() must be modified to specify there are 3 arguments on the parameter list.

    add_filter('img_caption_shortcode', 'my_img_caption_shortcode_filter',10,3);
    
    /**
     * Filter to replace the  shortcode text with HTML5 compliant code
     *
     * @return text HTML content describing embedded figure
     **/
    function my_img_caption_shortcode_filter($val, $attr, $content = null)
    {
    	extract( shortcode_atts( array(
    		'id'	=> '',
    		'align'	=> '',
    		'width'	=> '',
    		'caption' => ''
    	), $attr ) );
    	
    	if ( 1 > (int) $width || empty($caption) )
    		return $val;
    
    	$capid = '';
    	if ( $id ) {
    		$id = esc_attr( $id );
    		$capid = 'id="figcaption_' . $id . '" ';
    		$id = 'id="' . $id . '" aria-labelledby="figcaption_' . $id . '" ';
    	}
    
    	return '<figure ' . $id . 'class="wp-caption ' . esc_attr($align) . '" style="width: '
    	. (10 + (int) $width) . 'px">' . do_shortcode( $content ) . '<figcaption ' . $capid 
    	. 'class="wp-caption-text">' . $caption . '</figcaption></figure>';
    }
    
  2. Skip to note content
    Contributed by Rinku Y

    Example: Let display custom length of post excerpt.

    if( ! function_exists( 'prefix_custom_excerpt_length' ) )
    {
    	function prefix_custom_excerpt_length( $length )
    	{
    		return 40;
    	}
    }
    add_filter( 'excerpt_length', 'prefix_custom_excerpt_length', 999 );
    

    By default, WordPress display 57 character. you can set custom length using above code. this time except length will be 40. it is a nice and easiest use of add_filter.

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