While the plethora of How-To guides on installing PHP/Apache may seem to make the process easy, the sheer number of them and the sight variations between them are enough to make me lose faith in all of them.
When I first started learning PHP, it only made sense to me to use the PHP Manual to install PHP (e.g. [man]install.windows[/man]). If you bought a Honda car and wanted to know how to use it, would you look at a service manual from Honda or one from Chevrolet? I got PHP from php.net, so it just made sense to me to go to php.net to learn how to install it.
As for the specifics, here's the four lines I added to my Apache's httpd.conf file:
LoadModule php5_module "C:/php/php5apache2_2.dll"
AddHandler application/x-httpd-php .php
AddHandler application/x-httpd-php-source .phps
PHPIniDir "C:/php/"
Now, when I go to upgrade PHP, I simply extract the new files into c:\php\, restart Apache, and browse to a phpinfo() readout to confirm everything went as planned.