With some quick googling I couldn’t find out if there was a reasonable stable version to run on a wamp install – any advice?
Just in case you haven’t found a good way to test PHP 7 yet, it’s now available by default on servers configured by ServerPilot. So, you can use a $5/month DigitalOcean server, connect it to ServerPilot, use ServerPilot’s one-click WordPress install, and test your plugin on PHP 7. DigitalOcean actually bills hourly and ServerPilot has a free plan, so it won’t cost much to test that way.
Possibly it’s due to how the stack is configured on the server. Maybe try comparing against a new Vultr server that’s using ServerPilot’s stack rather than Vultr’s WordPress image which probably uses mod_php. That is, use this: https://serverpilot.io/wordpress-hosting
If you decided to change the field names through the WordPress options, be sure to change the field names in the code you added to comment.php (and, after correcting the field names in the code you added, be sure to refresh/clear browser cache).
