apply_filters() WP 1.0

Call the functions added to a filter hook.

The callback functions attached to filter hook $tag are invoked by calling this function. This function can be used to create a new filter hook by simply calling this function with the name of the new hook specified using the $tag parameter.

The function allows for additional arguments to be added and passed to hooks.

// Our filter callback function
function example_callback( $string, $arg1, $arg2 ) {
	// (maybe) modify $string
	return $string;
}
add_filter( 'example_filter', 'example_callback', 10, 3 );

/*
 * Apply the filters by calling the 'example_callback' function we
 * "hooked" to 'example_filter' using the add_filter() function above.
 * - 'example_filter' is the filter hook $tag
 * - 'filter me' is the value being filtered
 * - $arg1 and $arg2 are the additional arguments passed to the callback.
$value = apply_filters( 'example_filter', 'filter me', $arg1, $arg2 );


____c_box_note note-mark
- Global. Array. `$wp_filter` Stores all of the filters.
- Global. Array. `$wp_current_filter` Stores the list of current filters with the current one last.
____


##### Return

Mixed. The filtered value after all hooked functions are applied to it.


### Usage

```php
apply_filters( $tag, $value );

$tag(string) (required)

The name of the filter hook.

$value(mixed) (required)

The value on which the filters hooked to $tag are applied on.

$var,...(mixed) (required)

Additional variables passed to the functions hooked to $tag.

Code of apply_filters: wp-includes/plugin.php ^{VER 4.9.8}

<?php
function apply_filters( $tag, $value ) {
	global $wp_filter, $wp_current_filter;

	$args = array();

	// Do 'all' actions first.
	if ( isset($wp_filter['all']) ) {
		$wp_current_filter[] = $tag;
		$args = func_get_args();
		_wp_call_all_hook($args);
	}

	if ( !isset($wp_filter[$tag]) ) {
		if ( isset($wp_filter['all']) )
			array_pop($wp_current_filter);
		return $value;
	}

	if ( !isset($wp_filter['all']) )
		$wp_current_filter[] = $tag;

	if ( empty($args) )
		$args = func_get_args();

	// don't pass the tag name to WP_Hook
	array_shift( $args );

	$filtered = $wp_filter[ $tag ]->apply_filters( $value, $args );

	array_pop( $wp_current_filter );

	return $filtered;
}

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From tag: Filters (hooks)

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