I have files: test.xml, test2.xml and test.xsl
and in xslt there is:

<xsl:value-of select="document('test2.xml')//root/@value"/>

and xslt_process($xh, $xmlFile, $xslFile works ok - there is a value taken via document() function in output

but when I'll assign data from test.xml and test.xsl to strings then in output of:
xslt_process($xh, 'arg:/_xml', 'arg:/_xsl', NULL, $args);
there is no value from file referenced via document() function.... :(