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 );

Signature

apply_filters( $tag, $value )
tag
 (string) The name of the filter hook.
value
 (mixed) The value on which the filters hooked to <tt>$tag</tt> are applied on.

Return

(mixed) The filtered value after all hooked functions are applied to it.

Source

function apply_filters( $tag, $value ) {
	global $wp_filter, $merged_filters, $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;
25 more lines...
WP Trac GitHub

Link here