Q:

How to convert an XPS file using PHP or Python?

How to convert an XPS file to BMP, JPG, OR PDF using PHP or Python?

A:

There is a library for reading XPS; libxps. No bindings for PHP or Python mentioned, though.
It is used by the Evince reader, which can presumably print to PDF.
On windows, you can view XPS documents, and if you install pdfcreator you can "print" them to PDF.

