While I don't know of any PHP-based tools specifically aimed managing a pile of help documents, but I'm sure there's stuff out there.
Xero's help front-end is built on the ExtJS component library. In fact, it's directly derived from the ExtJS API Docs. So getting a slick UI is pretty easy.
It should be fairly easy to reverse-engineer how the client/server conversation is going, and come up with a server-side program to provide the web services the help-center code would need. It seems like it just manages some heirarchical data (for the help topic tree), and just loads what could be static HTML for the actual article content. Zend's MVC stuff makes implementing such services pretty straight-forward.
I would guess you could probably get a minimally functional help system written in a few days.
Of course, you might spend more time building an administration tool for your writers to use to create and organize all the help articles.