Q: Using HOST_WWW_ROOT on the MAMP stack I'm currently trying to teach myself PHP and MySQL using the MAMP stack.  Everything has been going good up until I came across this problem.  
I am reading a book and completing the exercises as I go and I'm at the point where I have to let the user upload an image and then store that image on the web server.  The book states that to do that, you make use of HOST_WWW_ROOT to set the location for the web files that will be uploaded.   
I have done exactly how the book says to, however it is not working.  
Is it possible to use this HOST_WWW_ROOT on localhost or can it only be used on a live server?

A: Every book will use something like SITE_ROOT or HOST_WWW_ROOT to refer to the location where your web server's public directory is located at.
MAMP FAQ:

Where should I store my HTML and PHP pages?
By default, PHP and HTML Pages should be stored inside the MAMP "htdocs" folder which is located in the MAMP Application directory /Applications/MAMP. This folder is called "Document Root". You can change the path for the Document Root in the MAMP application's Preferences Panel:


Now, depending on the platform you are developing your application on, some books would use something like SITE_ROOT, WWW_PUB, WWW_ROOT, etc. but they all refer, as you said, to the location where your 'web files' are.
See if there are some side notes indicating the book conventions.
