mysql binVars異常

        $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會衝突，先綁定:currency會使:currency_value改變，之後無法綁定:currency_value。