Saxon/C by Michael Kay.
From the webpage:
Saxon/C is an alpha release of Saxon-HE on the C/C++ programming platform. APIs are offered currently to run XSLT 2.0 and XQuery 1.0 from C/C++ or PHP applications.
BTW, Micheal’s Why XSLT and XQuery? page reports that Saxon is passing more than 25,000 tests for XQuery 3.0.
If you are unfamiliar with either Saxon or Michael Kay, you need to change to a department that is using XML.