try this code instead:
<?php
if(!session_id()) session_start();
echo "eWay Full Request: ".(isset($_SESSION["eWAY_XML_postedValuest"])?$_SESSION["eWAY_XML_postedValues"]:"n/a")."<br />";
echo "eWay Full Response: ".(isset($_SESSION["eWAY_XML"])?$_SESSION["eWAY_XML"]:"n/a")."<br /></p>";
}?>