I gave up on configuring Bitnami NGINX with a custom PHP app and re-did the installation into /opt/bitnami/nginx.html.
Now, my main issue is that the primary page that loads is the default Bitnami index.html. If I delete this page, my custom index.php does not load, and gives a 403 Forbidden. It is not a file permissions issue. Other .php files in the root folder load OK.