The form builder creates a .html file, to which I then add php code to provide dynamic content. When I invoke the .php file at the root level of the directory tree, it presents the original .html file to the browser, rather than presenting my new .php file. Therefore, I have to configure the URL to my form to point to form_name/form_name.php, rather than to form_name.php (i.e., the php file at the root level of the directory tree). This causes other problems downstream, for example, if a user were to go back a page in their browser, they would invoke form_name.php (at the root level), which would then present the original .html file, rather than the modified file with the added .php. How can I, instead, direct the root level .php file to the modified file with the added .php?

The Form Builder wasn't made to work with custom PHP. Having said that, there may be ways to change that. Wait for a user named Eric...

Change your form name extension from myformname.html to myformname.php.
Then I think it was line 62 of the formpage.cls.php
Change this

to this