So this is a guide for how to use XAMPP to test any PHP website on your own computer. Well, actually, it's more a guide of guides than an actual guide. I personally didn't find the process to be bad, but it also wasn't straightforward, and it involved a lot of googling separate problems. There was no One Guide for the Entire Process, but a bunch of separate things.
So I decided to put everything together. Here are the steps:
Step 1: Install XAMPP. Download from here.
Step 2: Set up XAMPP. Follow instructions here.
Step 3: Go to PhpMyAdmin and create any relevant MySQL tables, if any.
Step 4: Change the password for PhpMyAdmin (login to PhpMyAdmin, and click “Users” at the top).
Step 5: Get unlocked out from PhpMyAdmin.
Step 6: Edit your database access script to reflect the new PhpMyAdmin credentials.
Step 8: If you're having users upload things, make sure your copies of folders where you want uploads to go are writable.