Ebay style category chooser with mootools, PHP and MySQL?

I am looking to create a similar category chooser to ebay, allowing a next level style selector, but built with mootools, php and selecting the categories from MySQL.

Any help or tips on where to look or how to start more than welcome...

isn't that a simple fly-out menu? you could even do this with pure CSS although i ended up doing something else recently - have a look here http://fragged.org/dev/mooflyout.php (my conceptual staging area) - the flyout is not in production just yet and probably needs more work (built on top of work i found on the web some time ago) but it's good enough as proof of concept that can get you started. just mouseover the items on the menu bar and look at the source, couldn't be simpler. there's an iframe shim to keep the height cross browser but i plan on removing it