The function xmlrpc_decode() does not decode properly NULL values in SOAP 1.1 at the first level.

The following example 

var_dump(xmlrpc_decode(xmlrpc_encode_request('method', array(array(NULL)), array('version'=>'soap 1.1'))));

will produce

array(1) {
  [0]=>
  array(1) {
    [0]=>
    NULL
  }
}

which is correct, but when NULL value is used at the first level of array like in the following example, the answer is incorrect

var_dump(xmlrpc_decode(xmlrpc_encode_request('method', array(NULL), array('version'=>'soap 1.1'))));

will produce

array(1) {
  ["xsi:null"]=>
  string(0) ""
}