Thanks for the answer!
I've tried to use this web service and after configurating it all I've got some problems,
this is the code:
$params['users'][0]['id']="68";
$params['users'][0]['city']="NY";
$client = new Zend_Http_Client('<my domain>/webservice/rest/server.php');
$client->setParameterPost(array('token'=>'<my token>','method'=>'core_user_update_users','users'=>$params));
$response = $client->request('POST');
$lastrequest = $client->getLastRequest();
file_put_contents('php://stderr', "\n lastrequest: ".$lastrequest);
file_put_contents('php://stderr', "\n response: ".$response);
Using this code everything seems to work, it doesnt' display any error message but it doesn't update at all...
Can you give me any clue of why is this happenning? Thank you very much!