I'm assuming you aren't actually passing through "" as the error indicates and that "" is actually the address of your bigip management interface. If you are passing through a correct hostname and user/pass, then it's most likely due to BIG-IP's self-signed certs. Other tool kits allow you to bypass hostname verification. I found this link that may be helpful to you:
http://stackoverflow.com/questions/8443618/disable-certificate-verification-in-php-soapclient
It shows doing something like this:
$context = stream_context_create(array(
'ssl' => array(
'verify_peer' => false,
'verify_peer_name' => false,
'allow_self_signed' => true
)
));
$client = new SoapClient(null, array(
'location' => 'https://...',
'uri' => '...',
'stream_context' => $context
));
Hope this helps...
-Joe