How can I force users to access my page over HTTPS instead of HTTP?

I've got just one page that I want to force to be accessed as an HTTPS page (PHP on Apache). How do I do this without making the whole directory require HTTPS? Or, if you submit a form to an HTTPS page from an HTTP page, does it send it by HTTPS instead of HTTP?

Here is my example:

http://www.example.com/some-page.php

I want it to only be accessed through:

https://www.example.com/some-page.php

Sure, I can put all of the links to this page pointed at the HTTPS version, but that doesn't stop some fool from accessing it through HTTP on purpose...

One thing I thought was putting a redirect in the header of the PHP file to check to be sure that they are accessing the HTTPS version:

if($_SERVER["SCRIPT_URI"] == "http://www.example.com/some-page.php"){  header('Location: https://www.example.com/some-page.php');}

But that can't be the right way, can it?