I am trying to create a php website on WordPress for the first time.
When I create a page, it creates a permalink which is of the form http://localhost/?p=123. I don't know if there is a corresponding file.

I've installed insert-php plugin to read php code. It works fine on a static page. But how do I include another php file? I want to include my_utilities which contains all the back-end functions, in login. It has a permalink 'http://localhost/?page_id=45'.
What to do?
include 'http://localhost/?page_id=45' doesn't work.
'http://localhost/?page_id=45'type stuff is there for users to read pages, things you want to use as script should be uploaded to your server and their folder structure path defined in yourincludefunction, eg.include('path/to/folder/my_utilities.php);