Turns out I did not change the root username at all, I was just typing in the wrong username on the phpmyadmin screen!
Anyway, I finally got into phpmyadmin, where I thought the PHP compiler would reside, but it turns out phpmyadmin is only for MySQL database administration.
Where is the PHP compiler? I am totally new to server-side programming (I'm currently a COBOL/DB2 programmer who is "switching" to web programming) and am starting a PHP class tomorrow. I googled "php compilers" and as expected there were many out there to choose from, but I just assumed that XAMPP would contain one, and now it doesn't.