php mktime() or uniqid()

Question

I am using one of these to create a serial key. I would like to know md5(uniqid()) or md5(mktime()) better to be use?

Which one has a less change to get a duplicated key?

Answer 1

md5(uniqid("", true)) is the better method. time() and mktime() are accurate to one second, so two executions within the same clock second would produce duplicate keys.