When a URL specifying only a directory is requested …
eg.: http://www.domain.com/directory/

Initially, Apache looks for file names by default (in the order they appear):

index.html
index.htm
home.html
The .htaccess directive “DirectoryIndex” can be used to override this behavior.

In your .htaccess file, type the following line:

DirectoryIndex index.php index.html

In the above example Apache is forced to look for index.php first and look for index.html only if there is no index.php file present.

“DirectoryIndex” can be used to make any file you would like to appear as default. For

For instance, you could have:

DirectoryIndex firstpage.html first.html first.php

In the above example ,apache  would first look for firstpage.html, first.html, and first.php, returning a directory listing if none of those files were present in the directory.