Are you using this .htaccess directive?
By default a Server will try to load either an index.php, index.html or index.htm file. This code below tells the Server to load the index.php file and not an index.html file. This would allow you to keep an index.html file in a folder and it would NOT load.
# DIRECTORY INDEX FORCE INDEX.PHP
# Use index.php as default directory index file
# index.html will be ignored will not load.
DirectoryIndex index.php index.html /index.php
To make the index.html file load instead of the index.php file you would change the code to this:
# DIRECTORY INDEX FORCE INDEX.HTML
# Use index.php as default directory index file
# index.html will be ignored will not load.
DirectoryIndex index.php index.html /index.html