Xpath retrieving attribute

There are xml of the form

TITLE
TEST
TEST2

As to TITLE, TEST and TEST2 question is not necessary.
TITLE obtained simply TEST and TEST2 using
$xml->xpath('//test:test1');

but how to get at:the attr and attribute using xpath? Or at least attribute? As I understand it you need to using @ but it doesn't work //test:test2@at:attr

Please tell me you're familiar with xpath