我如何强迫用户通过HTTPS而不是HTTP访问我的页面?

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?I've got just one page that I want to force to