When should you create a new directory, rather than a file?

Suppose, for example, you have a website which has a user log in functionality. With respect to the root directory, would you host this page as login.php or /login?

What about a "Contact Us" page? Would you host this as contact.php or /contact?

Is there a general rule of thumb as to when to host a page directly in the root directory, and when to host it in its own sub-directory?