RewriteEngine On
RewriteCond %{HTTPS} !=on
RewriteRule ^/?(.*) https://%{SERVER_NAME}/$1 [R=301,L]

RewriteRule ^(.*)\.htm /$1.php [R=301,L]
RewriteRule ^(.*)\.html /$1.php [R=301,L]

This rewrite will not work on WordPress.  It forces .htm and .html files that no longer exist to go to the new .php versions of the pages.

Force Rewrite for https and .html to .php in htaccess

php, html, htaccess

Apache