You need to add the following lines to httpd.conf (If you're using Apache for Win32)
ScriptAlias /php/" c:/path-to-php-dir/"
AddType application/x-httpd-php .php
Action application/x-httpd-php "/php/php.exe"
Then restart Apache and it should interpret php files correctly.
For more information on installing Apache with php look at the following manual page: http://www.php.net/manual/en/install.apache.php