Exception Missing required key in single structure: users

by Jesus Marquez.  

I require to consume function core_user_create_users with next code:

$serverurl = $domain . '/webservice/rest/server.php'. '?wstoken=' . $token . '&wsfunction='.$function_name;

$user_fields = array();

$n='1';

$user_fields[0]['username'] = 'testusername'.$n;

$user_fields[0]['password'] = 'testpassword'.$n;

$user_fields[0]['firstname'] = 'testfirstname'.$n;

$user_fields[0]['lastname'] = 'testlastname'.$n;

$user_fields[0]['email'] = 'testemail'.$n.'@moodle.com';

$n='2';

$user_fields[1]['username'] = 'testusername'.$n;

$user_fields[1]['password'] = 'testpassword'.$n;

$user_fields[1]['firstname'] = 'testfirstname'.$n;

$user_fields[1]['lastname'] = 'testlastname'.$n;

$user_fields[1]['email'] = 'testemail'.$n.'@moodle.com';

$curl_post_data = array('users'=>$user_fields);

$str = http_build_query($curl_post_data);

        $curl=curl_init($serverurl . $restformat);

curl_setopt($curl, CURLOPT_POST, true);

curl_setopt($curl, CURLOPT_POSTFIELDS, $str);

curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);

curl_setopt($curl, CURLOPT_HTTPHEADER, array('Content-type: application/x-www-form-urlencoded'));

$curl_response = curl_exec($curl);

if ($curl_response === false) {

$info = curl_getinfo($curl);

curl_close($curl);

die('error occured during curl exec. Additioanl info: ' . var_export($info));

}

curl_close($curl);

print_r($curl_response);

But I get received this exception:

<EXCEPTION class= "invalid_parameter_exception">

<ERRORCODE>invalidparameter</ERRORCODE>

<MESSAGE>Detectado valor de parámetro no válido</MESSAGE>

<DEBUGINFO>Missing required key in single structure: users</DEBUGINFO>

</EXCEPTION>

Any help!