Why Cannot Get Other Attributes Except For Id When Using Api?

[Ruoke]

try
{
	$webService = new PrestaShopWebservice(PS_SHOP_PATH, PS_WS_AUTH_KEY, DEBUG);
	$opt['resource'] = 'customers';
	
	// Call
	$xml = $webService->get($opt);
	echo "<pre>";
	echo var_dump($xml);
	
	// Here we get the elements from children of customers markup "customer"
	$resources = $xml->customers->children();
	
	if (isset($resources)){
		foreach ($resources as $resource)
		{
			echo var_dump($resource);
                        echo $resource->attributes();

		}
	}
	
}

And the result is

object(SimpleXMLElement)#2 (1) {
  ["customers"]=>
  object(SimpleXMLElement)#3 (1) {
    ["customer"]=>
    array(3) {
      [0]=>
      object(SimpleXMLElement)#4 (1) {
        ["@attributes"]=>
        array(1) {
          ["id"]=>
          string(1) "1"
        }
      }
      [1]=>
      object(SimpleXMLElement)#5 (1) {
        ["@attributes"]=>
        array(1) {
          ["id"]=>
          string(1) "2"
        }
      }
      [2]=>
      object(SimpleXMLElement)#6 (1) {
        ["@attributes"]=>
        array(1) {
          ["id"]=>
          string(1) "3"
        }
      }
    }
  }
}

There is nothing but "id" in the "@attributes", how can I get other data (e.g, last name, first name)? Thank you. Besides, it works well with the filter, that is when the given last name is set to the request, it only returns the specific customer, not all the customers.  

Edited January 7, 2016 by Ruoke (see edit history)