After researching this on SO and google for hours now... I hope to get some help here: (I am just one step away from running a regex to remove the namespaces completely)
The multiple namespaces are messing with it, adding the following works for me
$response->registerXPathNamespace("site", "http://webservices.site.com/definitions"); $_res = $response->xpath('//site:SessionId');
also, see this previous stack overflow question