如何使用PHP和MySQL基于当前表行生成随机唯一ID？

I have a table that has an auto-increment "id". I have another column in the same table that has a "key", which I want to be based on the auto-increment "id". This will ensure that there is never a duplicate "key". I want to preferably have a 6 character unique id generator. I found this I have a table that has an auto-increment "id".