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

I'm late but I'm here with some good research data based on the functions provided by Scott's answer. So I set up a Digital Ocean droplet just for this 5-day long automated test and stored the generated unique strings in a MySQL database.

During this test period, I used 5 different lengths (5, 10, 15, 20, 50) and +/-0.5 million records were inserted for each length. During my test, only the length 5 generated +/-3K duplicates out of 0.5 million and the remaining lengths didn't generate any duplicates. So we can say that if we use a length of 15 or above with Scott's functions, then we can generate highly reliable unique strings. Here is the table showing my research data:

Update: I created a simple Heroku app using these functions that returns the token as a JSON response. The app can be accessed at https://uniquestrings.herokuapp.com/api/token?length=15

I hope this helps.