Pointing an HTML form "Action" to a php script file which is inaccessible via a URL

Can this be done? I want to point the "action" of an HTML form to a php script which does not have a URL. This script resides in a directory above the web root. I can access it via an "include" directive using various means but cannot access the script via the "action" method of a form. Is this even possible, or MUST the script be web-accessible in this case?

My reason is pure curiosity. It's not a homework assignment (I'm 64) nor is it for my work (I'm retired). I was once a programmer for an engineering firm but that was long ago and I have CRS syndrome now. :-)

Thanks in advance.