Consider the following rules
RewriteRule ^index\.html$ index.php [L]
RewriteRule ^index\.php$ index.html [R=301]
I want .html to use the .php file, but I want the url to have the .html extension always.
The above rule creates a redirection loop. What's the right way to do this?
You can either check against the actual request, or prevent looping entirely.
Option 1, check against request:
RewriteCond %{THE_REQUEST} ^[A-Z]{3,9}\ /index\.php
RewriteRule ^index\.php$ index.html [R=301]
This makes it so if the actual request isn't index.php, the redirect won't happen.
Option 2, prevent rewrite engine from looping entirely. Add this to the top of your htaccess file:
RewriteCond %{ENV:REDIRECT_STATUS} 200
RewriteRule ^ - [L]
The downside with this is that you may have rules that actually want to loop.
%{THE_REQUEST} var is literally the first line of the HTTP request, e.g. GET /index.php HTTP/1.1, and the ^[A-Z]{3,9} matches the GET part, which is the "method" and can be anywhere between 3 to 9 characters long. If you only care about GET/POST, you can alternatively change that to (GET|POST).