php 静态函数中$this的问题

class A{

    public $age = 50;
    private $money = 2000;
    static public $head = 1;
    public function tell(){
            echo $this->age,'
';
            echo self::$head,'
';
    }
    static public function sayMoney(){
            echo $this->money,'
';
    }

}
class B extends A{

    public $age = 22;
    private $money = 10;
    public function subtell(){
            parent::tell();
            echo $this->age,'
';
    }
    public function subMoney()
    {
            parent::sayMoney();
            echo $this->money,'
';

    }

}

$b = new B();
$b->subtell();//22 1 22;

echo '

---

';
$b->subMoney();

最后一句话报错Using $this when not in object context
但在调用subMoney()时不就绑定了$this,$this指向b对象，之后执行parent::sayMoney();由于静态调用，所以不绑定$this.在sayMoney（）执行的时候不应该得到2000吗，为什么会报错，和前面$b->subtell();调用有啥不一样