Calls the callback functions that have been added to a filter hook.

This function invokes all functions attached to filter hook `$hook_name`.
It is possible to create new filter hooks by simply calling this function,
specifying the name of the new hook using the `$hook_name` parameter.

The function also allows for multiple additional arguments to be passed to hooks.

Example usage:

// The 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
* that’s hooked onto `example_filter` above.
*
* – ‘example_filter’ is the filter hook.
* – ‘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 );

function apply_filters( $hook_name, $value, ...$args ) {
	global $wp_filter, $wp_filters, $wp_current_filter;

	if ( ! isset( $wp_filters[ $hook_name ] ) ) {
		$wp_filters[ $hook_name ] = 1;
	} else {
		++$wp_filters[ $hook_name ];
	}

	// Do 'all' actions first.
	if ( isset( $wp_filter['all'] ) ) {
		$wp_current_filter[] = $hook_name;

		$all_args = func_get_args(); // phpcs:ignore PHPCompatibility.FunctionUse.ArgumentFunctionsReportCurrentValue.NeedsInspection
		_wp_call_all_hook( $all_args );
	}

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

		return $value;
	}

	if ( ! isset( $wp_filter['all'] ) ) {
		$wp_current_filter[] = $hook_name;
	}

	// Pass the value to WP_Hook.
	array_unshift( $args, $value );

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

	array_pop( $wp_current_filter );

	return $filtered;
}