There will be a line in httpd.conf that looks something like this:
DirectoryIndex index.cfm index.htm index.pl index.html index.cfml index.jsp index.asp default.asp default.aspx index.aspx index.cgi
That line lists, in order of priority, all the files it will look for if you give just a directory, but no file name in the URL. Just add index.php to that list, and off you go!
Peace,
phidauex