Custom rewrites for pages and categories

If you want to have domain.tld/PAGE-NAME.php then you could just use %postname%.php as custom structure.

// Edit

As this approach seems to work for posts only, you might have a try with the structure /%postname%/. In addition, you put the following lines to your .htaccess:

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteRule ^(.*).php /$1/ [R=301,L]
</IfModule>

// Edit, again

Replacing in the plugin file of .html on PAGES each occurrence of “html” with “php” AND having set permalinks to /%postname%.php should do the trick. I have not checked the plugin for deprecated functions and the like – it works, though.