When writing a stylesheet that uses a callback function be sure to include a namespace declaration for php, as follows:
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:php="http://php.net/xsl" version='1.0'>