How to generate a random serial number

[eluser]Unknown[/eluser]
[quote author="jedd" date="1293352821"]Hi cleos and welcome to the CI forums.

You've got a pattern problem there ... [/quote]

Would you please explain to me the concept of cheap vs expensive when it comes to code and why md5 is cheaper than mt_srand. I might be wrong but I thought that I first would need an integer and then encode it with md5 which take us to square one. Also, about the pattern, why do you think it is a problem to contrast $rand_num with the db in the same function.

Thanks a lot for your help, thanks to this forum, I have learn so much about CI and I have still a lot to cover.