simple xml add namespaced child

I'm using SimpleXML to create an RSS feed for Google Products and I want to create a namespaced child but when I do for example

$item->addChild('g:id', 'myid');

it adds

<id>myid</id>

instead of

<g:id></g:id>

Besides I have added at the top

<rss xmlns:g="http://base.google.com/ns/1.0" version="2.0">

How can I add namespaced children?