Problem with $this in php static function

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();

The last sentence reports an error Using $this when not in object context
But when subMoney() is called, $this is not bound, $this points to the b object, and then executed parent::sayMoney(); is not bound to $this because it is called statically. Shouldn't it get 2000 when sayMoney() is executed? Why does it report an error? Is it different from the previous $b->subtell(); call? Same