final class TaskManager
{
    private static $tm = null;
    private function construct()
    {
    }
    public static function getInstance()
    {
        if (static::$tm == null) {
            static::$tm = new TaskManager();
        }
        return static::$tm;
    }
}
// final 让这个类不能被继承、让方法不能被修改 static 设置静态方法或属性的关键字