I still don't know why this happened, but having time constraints, I uninstalled SunOne ASP, PHP and Apache and re-installed them (Apache first, then PHP as an Apache module, then SunOne ASP). Everything is working now, including error messages.
I had upgraded PHP 5.0.2 to PHP 5.0.3, so I might have overlooked something that caused errors not to display. With a clean install, everything seems to work.
If anyone wants to comment, others may find this info useful for PHP upgrades.