Download the Windows PHP binary package from http://www.php.net/downloads.php
Extract to a temporary directory
Copy the following files to the directory you want PHP to live (paths with spaces are not recommended, ie c:\program files)
php.ini-recommended (rename to php.ini) - php.ini-dist can be used depending on your requirements php4apache2.dll or php5apache2.dll php4ts.dll or php5ts.dll
Modify php.ini to have the following doc_root = c:\apache2\htdocs (or wherever your document root for PHP happens to be)
Add your PHP home directory to the Windows PATH (eg, c:\php)
Add the following to [apache directory]/conf/httpd.conf (modify as required)
For PHP 4 do something like this:
LoadModule php4_module "c:/php/php4apache2.dll" AddType application/x-httpd-php .php
For PHP 5 do something like this:
LoadModule php5_module "c:/php/php5apache2.dll" AddType application/x-httpd-php .php
configure the path to php.ini
If you are using PHP override settings, make sure overrides are allowed in your Apache httpd.conf configuration file. There should be an AllowOverride command.
Note: you only need a small subset of the files in the download if you just want to run PHP.