Dreamweaver works on local files.  To edit any website (even those not made in DW - after all, it's all HTML), you would first need to have a DW site defined with a local root folder, then you'd need to get the files (and their dependent files) into that local root folder, and finally you can then edit those files using DW.  Ordinarily, this requires some FTP connection to the remote site so that you can not only GET the remote files but also PUT them back once you have edited them.  Dreamweaver has the ability to do this - when you define your site you also specify the FTP connection details.

I suggest you also use DW's F1 Help to read about defining a site.

See the following questions:

1. First, tell us what your skill level is with both HTML/CSS and PHP, please. 
2. Next, have you got Apache and PHP running locally? 
3. Do you have MySQL running locally? 
4. Have you defined localhost as your testing server? 
5. Do you have a remote site defined?

Dependent files are files that are required for any given page to function properly.  For example, a linked CSS file, or a linked javascript file, or an include file, or even an image displayed on the page would all be considered dependent files of the page.

In your Dreamweaver site definition, have you defined localhost as your testing server?  What do you get when you create a new PHP page in Dreamweaver, enter the following code on it -

<?php phpinfo(); ?>

and click the Live button on the DW toolbar?

Getting there.  Sounds like your site definition is properly set.

What edits are you wanting to make to PHPFOX?  Have you tried opening one of the pages and looking at/changing its code?