Where can I find documentation on the list of features that the soap client accepts?

For example, I just found out you can force it to pass single element arrays. Where is this feature documented and where can I read about other features that are supported?
$x = new SoapClient($wsdl, array('features' => SOAP_SINGLE_ELEMENT_ARRAYS));

In this particular example it appears null still gets passed back as a single element array which seems wrong.