Problem with $this in php static function

<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>

<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>