this is the responsibility of your webserver, I'm assuming you are using apache
look in your apace config file, there should be a line like:
#
# DirectoryIndex: sets the file that Apache will serve if a directory
# is requested.
#
# The index.html.var file (a type-map) is used to deliver content-
# negotiated documents. The MultiViews Option can be used for the
# same purpose, but it is much slower.
#
DirectoryIndex index.html
you can add on default files for a directory like such
DirectoryIndex index.html index.php mail.html main.php
if you type in http://mysite/mydir/
apache will first try and locate an index.html, if not found an index.php, and so on and so forth
