Foxycart Snippet for HMAC SHA256 Validation

I'm trying to create a snippet that will sign all forms and links within a chunk (or directly in the parameter) utilizing the Foxycart form validation script ( https://github.com/FoxyCart/FoxyCart-Cart-Validation--PHP/blob/master/foxycart.cart_validation.php )

However, I don't know much php and I'm not sure how to go about this. It would only need to sign one form of one product at a time. I know "signing of all links and form elements in a block of HTML" is possible just from comments in the script, but I have no idea how to turn that into a snippet. I have a client/friend who wants a simple web shop and this seems the best option at the moment. Not to knock Modx, but easy eCommerce isn't really there yet.

Has anyone else done this? I know foxycart integration without form validation is easy with Tv's, but I would like to secure it as well. Thanks!