I found a paper on MSDN which says it does work with PHP.
You can wrap up the ink dlls, and see them as a COM object which means you should be able to get at them from the HTML rendered by the PHP (also using JavaScript to process what's written on the client-side so you can pop it back to the server-side database).
It also suggests you can run as an ActiveX object from HTML.
Obviously much to learn here and from the responses above a bit bleeding edge for now.