There should be no path issues if you follow the path below:
For PHP file: [
localhost]
For HTML file: [
localhost]
The above paths are asuming that your file name is 'Index'. If you just type: "[
localhost]; then it may pick-up the HTML file as default, but it may notpick-up the php file as default. In which case, a simple fix would be to place a redirection script on the HTML file to redirect to the PHP file.
Now another issue could be that your PHP 4/5 is not working correctly, a simple way to test this would be to install a simple free PHP script like 'OSCommerce' and testing the PHP through a script like that which have many help files/forums.
Or if you are an advanced user, you could go into the core-files of Wamp-Server and fix the PHP if it is corrupt, a even simpler way to fix PHP would be to reinstall WampServer.
I hope this helps, remember, I am no expert...
:-)