I'm not totally sure but pretty sure that you do need to recompile Apache to recognize PHP.
Apache is supposed to have support for dynamically loaded modules in 1.3.12, but as far as I've heard it's not operating smoothly yet.
When you do compile Apache to use with php, I know I did the --with-php=/usr/local/php option, so I expect that it's necessary.
btw ...
For a GREAT guide on installing PHP, MySQL, Apache, and a bunch of other things in nice harmonious working order, got to devshed.com, and check the PHP section of the site for the article on it.
-john