WP_Hook::add_filter() WordPress Method

	public function add_filter( $hook_name, $callback, $priority, $accepted_args ) {
		$idx = _wp_filter_build_unique_id( $hook_name, $callback, $priority );

		$priority_existed = isset( $this->callbacks[ $priority ] );

		$this->callbacks[ $priority ][ $idx ] = array(
			'function'      => $callback,
			'accepted_args' => $accepted_args,
		);

		// If we're adding a new priority to the list, put them back in sorted order.
		if ( ! $priority_existed && count( $this->callbacks ) > 1 ) {
			ksort( $this->callbacks, SORT_NUMERIC );
		}

		if ( $this->nesting_level > 0 ) {
			$this->resort_active_iterations( $priority, $priority_existed );
		}
	}