<?php
$dom = new DOMDocument();
$dom->loadXML('<html xmlns:some="some:ns" some:test="a" test2="b"/>');
var_dump($dom->documentElement->getAttributeNames());
?>
array(3) {
[0]=>
string(10) "xmlns:some"
[1]=>
string(9) "some:test"
[2]=>
string(5) "test2"
}