Developing webservice - 'do not match signature' error

Using Moodle 2.3.1

With this simple setup :

$token = '6aa701162583677b5247683d127d2a80';
$domainname = 'http://localhost/moodle';

/// FUNCTION NAME
$functionname = 'moodle_user_get_users_by_id';

$userids = 3;
$params = array( 'userids' => $userids );

///// XML-RPC CALL
header('Content-Type: text/plain');
$serverurl = $domainname . '/webservice/xmlrpc/simpleserver.php?wsusername=testeur&wspassword=Test-2012';
require_once('./curl.php');
$curl = new curl;

$post = xmlrpc_encode_request($functionname, $params);

$resp = xmlrpc_decode($curl->post($serverurl, $post));

print_r($resp);

I get this response :

Array
(
    [faultCode] => 28194976
    [faultString] => Calling parameters do not match signature | ERRORCODE: 
)

Where am i wrong ?

To answer the question: why don't use core_user_get_users_by_id ? because i got : Method "core_user_get_users_by_id" does not exist

To answer the question: why simpleserver.php and not server.php with token ?

it's because with Client test webservice and no token, it works well, and in Client test webservice with token i got this cool error : 

exception 'Zend_XmlRpc_Client_FaultException' with message 'Failed to parse response' in C:\wamp\www\moodle\lib\zend\Zend\XmlRpc\Client.php:370
Stack trace:
#0 C:\wamp\www\moodle\webservice\xmlrpc\locallib.php(134): Zend_XmlRpc_Client->call('moodle_user_get...', Array)
#1 C:\wamp\www\moodle\admin\webservice\testclient.php(154): webservice_xmlrpc_test_client->simpletest('http://localhos...', 'moodle_user_get...', Array)
#2 {main}