I'm planning on coding a Business Directory - along the lines of
Yahoo's Yellow Pages at http://yp.yahoo.com (and ones at many other
sites.)

After some searching on the web using various combinations of "yellow
pages" + "business directory" + mysql + "data structure" I wasn't able
to come up with much.

The Yahoo YP have some interesting features (a few I detail below),
which makes their data tree do things such as jump back to an earlier
level, and I'm starting to think how such things can be implemented
in a data structure.

Before I start to code such an application - although not an overly
complex task, to avoid reinventing the wheel, and to save a few hours -
I'm wondering if anyone knows of any existing articles discussing the
implementation of such a system, taking into account the features I'm
just about to mention?

With the Yahoo YP, there are top-level categories such as "Automotive",
"Home and Garden", "Real Estate" etc. and then sub-categories, such as
"Automotive Repair". Further down, just before you are shown the
businesses, you have final categories, which I will call
container-categories, such as "Bars", "Video Rental Stores" etc. that
hold the actual businesses. Extending the Automotive category started
above, a full category path might be Automotive -> Repair ->
Transmissions.

So, a couple of features I noted (which I would like to implement):

(1) One feature with Yahoo YP, is that a business can sometimes be
found in more than one final container-category. For example, the
business "BJ's Chicago Style Brewpub" can be found in BOTH the
container-categories "Bars" and also in "Restaurants."

(2) Another feature - categories themselves can belong to multiple
higher level categories. For example, the category "Bars, Pubs, and
Clubs" can be found within both the higher level categories of
"Entertainment/Arts" AND "Food & Dining."

And another example, "Bakeries" is both located at:

Top -> Food and Dining -> Restaurants -> Bakeries

AND ALSO

Top -> Food and Dining -> Bakeries

(Interestingly, in the final example - the "page locator" text in both
instances displays as "Top > Food and Dining > Restaurants > Bakeries"
in both cases.)

FYI, I will be coding up in PHP 4.3.8 and MySQL 4.0.20.

Any advice, and/or pointers to articles would be most appreciated.

Thanks in advance for any replies.

Miner2049er.