I think my issue was either 1 of 2 things.
1.) after installing the new php 5 module (linux/apache/apxs), the apache httpd.conf config file showed both modules (rather than having 1 of them commented out).
2.) I was running an old version of the zend-optimizer.
I upgraded my zend-optimizer and commented out the "LoadModule" line for php4 in httpd.conf, and everything worked after that. I should have done 1 step at a time to figure out which was causing the problem, but was in a rush.