public static function apply($key, $callback, $default = NULL) {
		$value = self::get ( $key, $default );
		if (is_string ( $callback ) && function_exists ( $callback )) {
			$value = call_user_func ( $callback, $value );
		} elseif (is_callable ( $callback )) {
			$value = $callback ( $value );
		} else {
			return $value;
		}
		return self::set ( $key, $value );
	}