The reason for setting expose_php to 'off' is to not publish the PHP version you are using. Automated vulnerability scanners created a database of servers running specific version and when an exploit comes out for a specific version, they immediately target servers which run that version.
To hide the index.php file, simply redirect all requests for non-existing files to index.php and let index.php decide how to handle the request (which can be found in $_SERVER["REQUEST_URI"]. Use this UrlToolkit rule for that:
UrlToolkit {
ToolkitID = generic_rewrite
RequestURI exists Return
Match .* Rewrite /index.php
}