To make your import dynamic, try this code:
<?php
$dom = new DOMDocument();
$dom->load('main.xsl');
$xpath = new DomXPath($dom);
$importnode= $questionsXsl->createElement('xsl:include');
$attr= $questionsXsl->createAttribute('href');
$attr->value = 'import.xsl';
$importnode->appendChild($attr);
$dom->documentElement->insertBefore($importnode,$ref);
$dom->loadXml($dom->saveXml());
?>
this code basically loads the main stylesheet, prepend the import xsl code then reload as xml string so the imported stylesheet will be loaded at dom.