[libvirt] LIBVIRT-PHP List of domains

Hi.. I am trying to get the names of all the xen domains using libvirt-php. I saw this php script: <?php $conn = libvirt_connect('null', false); $doms = libvirt_list_domains($conn); print_r($doms); ?> It gives me error. Could you please tell me how to connect to xen hypervisor? I have also tried: $conn = libvirt_connect('xen:///', false); but it wont work..