您在正确的轨道上，使"NULL"成为字符串.如果需要将其设置为NULL，则删除$ParentEventID周围的qoutes.

You are on the right track making "NULL" a string. If you need to set it NULL, then remove the qoutes around $ParentEventID.

mysql_query("UPDATE events SET ParentEventID = $ParentEventID");

但是，在执行此操作之前，请确保$ParentEventID === NULL的值.

However, before doing so make sure that the value of $ParentEventID === NULL.