$this ใน class ใช้ไม่ได้ มันบอก Using $this when not in object context

class cart {
	
	/**
	* add($customer_id,$session_id,$product_id,$quantity) เพิ่มสินค้าลงตะกร้า
	*/
	static public function add($customer_id, $session_id='', $product_id, $quantity='1') {
		global $db;
		$sql = "select * from "._dbprefix_."cart where";
		if (!empty($customer_id)) {
			$sql .= " customer_id = ".mydb::psql($customer_id)."";
		} else {
			$sql .= " cart_session = '$session_id'";
		}
		$rs = $db->execute($sql);
		if (!$rs->EOF) {//shopping cart exist
			
		} else {// shopping cart is not exist
			$this->newcart($customer_id, $session_id, $product_id, $quantity);//บรรทัดเนี้ย
		}
	}//add
}