How can I reuse the code to capture a param in a URL and place in a value in a hidden form?

I have a form that will be re-used in different posts.

I also have php code that will be reused to populate the form with the value in the URL.

I tried to use php shortcode and wp php exec, but it doesn’t appear to work:

   value = "[exec]echo htmlentities($_GET['source'])[/exec]"

I basically want a simple, reusable way to grab parameters and have it as the value in a hidden field on a form.