php 靜態函數中$this的問題

class A{

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

}
class B extends A{

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

    }</code>

}

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

echo '

---

';
$b->subMoney();

最後一句話報錯Using $this when not in object context
但在呼叫subMoney()時不就綁定了$this,$this指向b對象，之後執行parent ::sayMoney();由於靜態調用，所以不綁定$this.在sayMoney（）執行的時候不應該得到2000嗎，為什麼會報錯，和前面$b->subtell();調用有啥不一樣