Rewrite rules for Smarty Templates on a www.ads-software.com site

At a site for a client, I have implemented some PHP templates (These are ‘Smarty’ Templates) for a client.

The issue I cannot seem to resolve is how to add rewrite rules for these:

/folder/some-file-calc.php to be /folder/some-file-calc/ (drop the .php and add a trailing slash), but only for this folder!

This is important as we will manually include these in our (soon to be invoked) SEO Plugin. We want a clear and semantic URL for these.

I have searched many forums including, of course, these for an answer and found many but none seem to be work (there seems to be a ‘flood’ of ‘bad’ info out there). The WP site itself uses only the default WP rewrite rules. The Calc app is installed at /folder/ and WP at root…

Anyone have any luck with a situation like this? Please let me know what has worked for you (not links to sites with bad or untested info.). I am open to any suggestions including other methods. Thanks!