I have not yet come across a legitimate need to re-process with
PHP/XSLT. Issues like this can usually be resolved by superior design.

I assume you are well aware that you can pass strings to the XSLT
processor when you call the xslt_process() function (see last argument).

so, $a = array("msg" => "this is a test");

could be passed to XSLT via

xslt_process(,,,,$a); // insert other params as required (can use NULLs)

and would be used in the XSLT like

<xsl:param name="msg" select="''" />

You MUST include the above line! The value will be overwritten with the
one supplied from PHP.