If you are processing your page each time via post you could pass your selected option as a $_POST variable and use:
http://us.php.net/manual/en/xsltproc...tparameter.php
basically in your xsl define a parameter
Code:
<xsl:param name="selectedOption">No Value</xsl:param>
then after your post set:
PHP Code:
$proc = new XSLTProcessor;
$proc->importStyleSheet($xsl); // attach the xsl rules
$proc->setParameter('', 'selectedOption', $_POST['selectedOption']);