If the script is going to be called from scratch each time - then it seems moot to try and speed up a single lookup. You do have other problems to consider though.

If the script will always be running, then it makes sense to have that information in memory if you can afford that memory. Then you run into the problem if multiple copies can be running at the same time. How do you share the memory? How do you avoid race conditions on files on the system.

You very well may want to consider a database instead. I can't offer any suggestions for how to do this as I don't do web/database programming. Consider for instance updating your hash and your external file. What happens if the system crashes after the user has been added, but before you have had a chance to update your text file.

There are a lot of sagely monks here that have probably done exactly what you are trying to do or something very similar. Take the time to explain the entire process and I am sure you will get the advice you need - if not working code.

Cheers and goodluck - L~R