怎么把返回的xml转成数组. 这个xml的结构第一次接触, google了一翻, 没有找到解决办法

<?xml version='1.0' encoding='UTF-8'?><ns1:getActivityOptionsResponse xmlns:ns1="http://www.ebay.com/marketplace/returns/v1/services"><ns1:ack>Success</ns1:ack><ns1:version>1.1.0</ns1:version><ns1:timestamp>2014-11-21T07:05:54.874Z</ns1:timestamp><ns1:activityOptions>ISSUE_REFUND</ns1:activityOptions></ns1:getActivityOptionsResponse>

$s =<<< XML
<?xml version='1.0' encoding='UTF-8'?><ns1:getActivityOptionsResponse xmlns:ns1="http://www.ebay.com/marketplace/returns/v1/services"><ns1:ack>Success</ns1:ack><ns1:version>1.1.0</ns1:version><ns1:timestamp>2014-11-21T07:05:54.874Z</ns1:timestamp><ns1:activityOptions>ISSUE_REFUND</ns1:activityOptions></ns1:getActivityOptionsResponse>
XML;

print_r(simplexml_load_string($s, null, 0, 'ns1', true));

SimpleXMLElement Object<br />(<br />    [ack] => Success<br />    [version] => 1.1.0<br />    [timestamp] => 2014-11-21T07:05:54.874Z<br />    [activityOptions] => ISSUE_REFUND<br />)<br /><br />