After some extra work, there's what I figured out as a solution myself:
<?php
$signatur = $_GET['signatur'];
# LOAD XML FILE
$XML = new DOMDocument();
$XML->load( 'xml/'.$signatur.'.xml' );
# START XSLT
$xslt = new XSLTProcessor();
# IMPORT STYLESHEET 1
$XSL = new DOMDocument();
$XSL->load( 'metainfo.xsl' );
$xslt->importStylesheet( $XSL );
#PRINT
print $xslt->transformToXML( $XML );
?>