对SQL查询进行序列化通常会导致性能下降.您可以在一条语句中完成全部操作:

Serializing the SQL queries often leads into bad performance. You can do all in one statement:

$sql = "update ".$billing_table." c ".
       "inner join dids d on d.did=c.telcom_num ".
       "set c.did_id = d.id, c.customer_id = d.customer_id ".
       "where c.customer_id = 0;";