Alright, so unlike other solutions I only want one subdomain to appear to be added.

Say I wanted example.com/folder/page.php to be displayed as foo.example.com/folder/page.php, how would I go about doing this?

The foo will never change, and it only needs to be for one page.

Various other sites have been unable to provide an answer, thanks in advance.

Edit: Oh, and I also have full DNS records to the domain.