Mere seconds after posting that last entry I noticed that the following in httpd.conf were commented out:

LoadModule php4_module libexec/httpd/libphp4.so
AddModule mod_php4.c

Uncommenting those has Apache recognizing .php files and everything working nicely.

Is there a Murphy's Law-esque axiom for publicly asking a question being the surest way to stumbling across the answer yourself?