PHP: How to generate a random, unique, alphanumeric string for use in a secret link?

Question

How would it be possible to generate a random, unique string using numbers and letters for use in a verify link? Like when you create an account on a website, and it sends y

Answer 1

Here is what I use:

md5(time() . rand());    
// Creates something like 0c947c3b1047334f5bb8a3b7adc1d97b