What PHP libraries would you recommend to implement the following:

• Three dependent drop-down lists
• Three XML data sources
• AJAX-based

Essentially, I'd like to create an XML database and wire up a form that allows the user to select three different dependent parameters:

1. User clicks Region
2. User clicks District (filtered by Region)
3. User clicks Station (filtered by District)

Even though I would like to use PHP and XML, the general problem is:

• One XHTML form
• Three dependent, cascading drop-down lists
• Three flat files (no relational database) for the list data

The solution must be efficient, simple, reliable, and cross-browser.

What technologies would you recommend to solve the problem?

Thank you!