adding a namespace when using SimpleXMLElement

This is what I am after

<!-- language: lang-xml -->
<ws:Test>
    <ws:somename2>somevalue2</ws:somename2>
    <ws:make>
        <ws:model>foo</ws:model>
        <ws:model>bar</ws:model>
    </ws:make>
</ws:Test>

This is my current code

<!-- language: lang-php -->
$xmlTest = new SimpleXMLElement('<Test/>', 0, false, 'ws', true);
$xmlTest->addChild("ws:somename2", "somevalue2", 'http://microsoft.com/wsdl/types/');
$make = $xmlTest->addChild('ws:make', null, 'ws');
#$make->addAttribute('name','Ford');
$make->addChild('ws:model', 'foo', 'ws');
$make->addChild('ws:model', 'bar', 'ws');
header ("Content-Type:text/xml");
print_r($xmlTest->asXML());

but it outputs

<!-- language: lang-xml -->
<Test>
    <ws:somename2>somevalue2</ws:somename2>
    <ws:make>
        <ws:model>foo</ws:model>
        <ws:model>bar</ws:model>
    </ws:make>
</Test>

As you can see the ws: is missing from Test