2
check your .htaccess file and make sure that your server will automatically show php files if named 'index'.
For example, my server is set up to look for html and php only, so when I inadvertently place index.htm in the directory, I get the same thing as you descibe, or a 404 error...as soon as I rename the file with the html extension everything goes back to being happy.

Without knowing anything about your set-up, what currently works, and what does not, that is where I would start.
Good Luck!