Here is a post in the Xara Knowledgebase FAQ section:

http://support.xara.com/index.php?_m...articleid=2765

Here is what it says:

The first page within your document will always be exported with either a .htm or .html extension. Web Designer allows you to export all subsequent pages with any file extension. Open the Web properties dialog and select the Page tab. Apply the page name and extension within the Page filename field.

Their customer support people sent me that link when I asked them if I could use php pages.