Add the following line of code to your .htaccess file:
DirectoryIndex index.php
Your .htaccess file can be found by enabling the viewing of hidden files. If there isn't one created already in public_html, you can create a new one yourself. If your index file is named differently than index.php, be sure to modify the code above as necessary.
to .htaccess . That way, if you do not have an index.xxx file in the directory, it will not display an directory listing page, but instead generate a "403 - Forbidden" message.