appending php code

This is a limitation you’ll hit with static sites in general – nothing can be dynamic, because then it wouldn’t be static 😉

I had a quick look at the post Robin made – if what her plugin collects is output as inline JS on the page (as it probably would be with wp_localize_script), it should work fine with Simply Static, although with the proviso that the code that is output would be the options at site generation time, and obviously wouldn’t be dynamic after that.

In general you wouldn’t be able to re-add dynamic functionality relying on WordPress with one PHP file, as you’d need the entire WordPress framework present to process it (so then, there’d be no point going static at all). Adding some custom code for some simple dynamic functionality would be fine, but you’d then also lose the benefit of being able to use super-fast static hosting options like AWS’ S3.

So it would kind of depend on your situation… having said all of that, it might even just be a missing script that you need to include in Simply Static’s settings! Are there any console errors – perhaps 404s on looking for scripts that Robin’s plugin uses? If not you might need to look at re-implementing the functionality in a purely frontend way, JS only.