mysql binVars异常

        $sql = "            UPDATE merchant            SET amount = :amount, currency = :currency, currency_value = :currency_value, orders_tag = :orders_tag            WHERE orders_id = :orders_id            ";        $sql = $db->bindVars($sql, ':amount', $amount, 'float');        $sql = $db->bindVars($sql, ':currency', $currency, 'string');        $sql = $db->bindVars($sql, ':currency_value', $currency_value, 'float');        $sql = $db->bindVars($sql, ':orders_tag', $zv_orders_tag, 'string');        $sql = $db->bindVars($sql, ':orders_id', $orders_id, 'integer');        $result = $db->Execute($sql);

:currency和:currency_value会冲突，先绑定:currency会使:currency_value改变，之后无法绑定:currency_value。