suggestions for mapping the layout / functionality of a website (like with UML)

I have just taken over a semi-large web development project. In an effort to familiarize myself with the project as a whole, I would like to create some kind of graphical layout of the website including the way the pages link to each other but also including how the php code works and the DB is layed out (like a flow chart but better). UML seems to be the key for this, unfortunately I know very little about it.

I have microsoft visio and I am sure it would do this, the problem I am having is that I am not sure HOW to do it. I took many programming classes in college and the only time we did anything like what I am thinking is when we did state machine diagrams. Is there some kind of data modeling scheme that is best suited for mapping a website and DB.

So, for some of you more professional programers out their that have acutally done some planing before you started coding, where can I go to learn about the best way to map out the web site? Should I use something other than visio (and for a different reason than M$ sucks).

Thank you in advance