RewriteEngine On
RewriteBase /

# Redirect /users/index.php to /users
RewriteCond %{REQUEST_URI} ^/users/index\.php$
RewriteRule ^users/index\.php$ /users [R=301,L]

# Rewrite /users to /users/index.php internally
RewriteCond %{REQUEST_URI} ^/users$
RewriteRule ^users$ /users/index.php [L]