In my previous post regarding getting started with Yahoo store, I remarked that PHP seemed to be the direction Yahoo was moving its store.  If that's the case, they're pretty mum on the subject.  I asked YS support the following question:

I'm starting to set up my site, and I see so much documentation on generating HTML pages and inserting store tags, which pretty much means I need to create a page for each item in my catalog (unless I use RTML templates).  There is very little on PHP, but apparently I can't use PHP and Store Tags.  Is that a problem?  Is is possible to create a database driven cart using PHP and, rather than store tags, the HTML output of the store tags, changing the proper variables (such as the category or item)?  I'm an experienced web developer, so this is a pretty basic concept for me, but there just isn't a lot of information to go on.

This was their reply:

Thank you for contacting Yahoo! Small Business Support.  I understand you are having trouble with store tags and apologize for any inconvenience this may have caused you.  You can use php pages and use store tags on that.But there is a small problem.When you use store tag on php page you cant directly put store tag on to php page.Instead create an item page with an extention .html and use store tags on that.Use IFRAME tag to bring that item page to php page.If you put store tag directly to php code there will be rendering
issues.  I am putting a sample skeleton of the code below

<?php
<IFRAME src="itempage.html"></IFRAME>
?>

Please use the below help link for store tags http://help.yahoo.com/l/us/yahoo/smallbusiness/store/tags/.  You can use your custom php scripts to create custom shopping cart.  But if you do that , you cant take advantage of shipping,payment option etc that is been provided by Yahoo! Merchant Solution plan.You dont need to be in Yahoo! Merchant Solution if you want to use custom PHP scripts and
create custom shopping cart.You can downgrade your plan to Yahoo! Web Hosting if you want to create custom shopping cart using php scripts. If you want to utilize tools provided by Merchant Solution you will have to use Yahoo!'s Check out.

So, long story short--there really isn't a way to develop a truly dynamic cart on a Yahoo! Store.  The best you can do is RTML templates, which do allow for a good deal of functionality.  But not a truly dynamic cart.