To inform Apache to load mod_php at startup time, httpd.conf must be edited and the following must be typed in:
LoadModule php4_module libexec/libphp4.so
underneath the Dynamic Shared Object (DSO) Support section. PHP may have already done this for you. Also, Apache must know when to use the mod_php module, therefore you will need to add this in the relevant section:
AddType application/x-httpd-php .php .phtml .php4 .php3
One may also wish to edit the DirectoryIndex directive in httpd.conf to:
DirectoryIndex index.html index.php index.phtml index.php4 index.php3
At this point, everything is all set. Restart Apache and view the error log to confirm the installation was a success.