It seems like your web server does not have "index.php" as the default file to open when you navigate to a directory. This is something you will have to fix in apache httpd.conf file.
Alternatively you can place an "index.html" file in the directory, and do html redirect by placing this in it:
Code:
<html>
<head><title>Redirecting ... </title>
<meta HTTP-EQUIV="REFRESH" content="0; url=index.php">
</head>
<body>
</body>
</html>
