PHPStorm自动提示方法

    /**
     * 一定要写@return static
     * @return static
     */
    public static function getInstance() {
        $className = get_called_class();
        if (!isset(self::$_instance[$className])) {
            self::$_instance[$className] = new $className();
        }
        return self::$_instance[$className];
    }