Okay jetzt kann ich das speichern...
$xmlClient = new SimpleXMLElement('<client/>');
$xmlClient->addChild('name', 'NAME1');
$xmlClient->addChild('salutation','HERR');
$xmlClient->addChild('first_name','FIRST');
$xmlClient->addChild('last_name','LAST');
$xmlClient->addChild('street','STREET');
$xmlClient->addChild('zip','ZIPPLZ');
$xmlClient->addChild('city','CITYYY');
$xmlClient->addChild('state','STAEEETE');
$xmlClient->addChild('country_code','DE');
$client = $xmlClient->asXML();
$client_objekt = $billoWrap->addClient($client);
$client_idnr = $client_objekt->client->id;
.....