Fatal error: Cannot access empty property

代码整合如下：

<?php 


abstract class Model{
    protected $table = NULL;
    protected $db = NULL;
    public function __construct(){;
        var_dump(MySQL::getIns());
        $this->$db = MySQL::getIns();
    }
}

class catModel extends Model{
    protected $table = 'emec_cat';
    public function addCat($date){
      $sql = $this->db->Autoexpr($date);
      return $this->db->qurey($sql);
    }
} 

class MySQL {

        static protected $Ins;
        protected $res;



        static function getIns(){
            if(self::$Ins instanceof self){
                return self::$Ins;
        }
            self::$Ins = new MySQL();
            return self::$Ins;
        }

        private function __construct(){
            $this->res = 1;
        }

        protected function __destruct(){
            mysql_close($link);
        }
}

$date = array();
$date['cat_name'] = $_POST['cat_name'];
$date['parent_id'] = $_POST['parent_id'] + 0;
$date['intro'] = $_POST['intro'];
$catMode = new catModel();
$catMode->addCat($date);
 ?>

会出现：
Fatal error: Cannot access empty property in F:\Apache2.2\htdocs\emec\test.php on line 9
去掉基类的abstract问题依旧。
环境:
apache2.2 php5.3.5

---

写多了一个$