What is the correct format for localhost URL?

I've installed MAMP to run a local test server.

This appears to be working correctly with the URL in the following format:

http://localhost/folder/filename.php/

Where folder is the folder for the site located inside the Apache Document Root Folder - as defined in the MAMP preferences.

I have web sharing turned off.

Looking around the web it seems that the format for the URL should be:

http://localhost/~username/folder/filename.php

However, when I insert "~username" in the URL the browser can't find the page.

These are my first tentative steps in setting up a local test server and so I'm concerned that something might be amiss!

I've hunted around and can't find any documentation that clarifies the issue.

Can anyone clarify the issue or point me to appropriate documentation.

Thanks.