Ok folks .. thanks for all the help. I did it this way:

1) A HTML-Page is passing a GET-parameter to a php5 page.

2) The PHP page is opening an xml-File and adds the parameter to the
DOM so that it can be used by the xsl:

$xml_src = 'searchdata_de.xml';

// Xpath-Querys
$parent_path = "//PAGES";
$next_path = "//PAGES/PAGE";

// Create a new DOM document
$dom = new DomDocument();
$dom->load($xml_src);

// Find the parent node
$xpath = new DomXPath($dom);

// Find next Child
$parent = $xpath->query($parent_path);

$next = $xpath->query($next_path);

// Create the new element
$element = $dom->createElement('SEARCHFOR', $searchfor);

// Insert the element before a Child
$parent->item(0)->insertBefore($element, $next->item(0));

3) The PHP is processing the new xml-data with an xsl and builds up
the html-document:

$xsl = new DomDocument;
$xsl->load('search_de.xsl');

$xslt = new XsltProcessor;
$xslt->importStylesheet($xsl);

$transformation = $xslt->transformToXml($dom);

echo $transformation;

Voila! ;-)