You want to add the mod_php scripting module to your existing Apache server on Windows.
This recipe needs to be described largely in terms of actions rather than explicit commands to be issued.
The PHP module installation on Windows requires a lot of nit-picky manual steps. To test that your installation was successful, create a file named info.php in your server's DocumentRoot; the file should contain the single line:
<?php phpinfo( ); ?>
After restarting your server, try fetching the document info.php from it using a browser. You should see a detailed description of the PHP options that are active.
There are numerous additional options and extensions available for PHP; the recipe given here is only the most basic installation. See the install.txt file in the PHP4 directory and the documentation on the web site for more details.