PHP - 编辑初始化的单例对象

Question

我有这门课，

class Player {

    protected static $volume;
    protected static $instance = null;

    protected function __construct()
    {
        self::$volume = 5;
        $this->maxVolume = 20;
    }

    protected function __clone() {}

    public static function getInstance() {
        if (!isset(static::$instance)) {
            static::$instance = new static;
        }
        return static::$instance;
    }


    public function volumeUp() {
        if (self::$volume < $this->maxVolume) {
            self::$volume = self::$volume + 1;
        }

        return static::$volume;
    }

    public function volumeDown () {
        if (self::$volume > 0) {
            self::$volume = self::$volume - 1;
        }

        return static::$volume;
    }
}

我想通过调用volumeUp或volumeDown方法动态更改对象值。

$player = \Player\Player::getInstance();

 if (isset($data['volume']) && isset($data['down'])) {

        echo $player->volumeDown();
    }

    if (isset($data['volume']) && isset($data['up'])) {
        echo $player->volumeUp();
    }

当我调用那些方法时，对象就像新实例一样，并且给我总是相同的结果。

我做错了什么？