I'm not using XAMPP. Instead, I'm using LAMP components installed individually on Debian Buster (at least for home computer). The Apache error log showed me the problem - apparently, utilities.inc.php was trying to load components automatically related to HTML_QuickForm2, and that created the failure conditions of my system trying to "load" HTML_QuickForm2 component classes. In fact, since I was using Debian locally, I could tell my the Pear HTML_QuickForm2 stuff was located in /usr/share/php. So, I commented out the class_loader() and spl_autoload_register lines in the utilities.inc.php and added require('classes/User.php') and require('classes/Post.php') manually when needed (and I did opt for the Post.php vice the Page.php class, but that's a trivial set of tweaks). And then, basically, it worked! So, the auto loading class feature was breaking HTML_QuickForm2.
If anyone knows how to keep the autoloading class feature, let me know. But since this OOP code is so small, I figure it's a small price to pay to manually load a class or two as needed.