We are setting xmlns attributes at the root element but PHP would
still attach redundant ones to the DOM elements created with `createElementNS`.
That was because PHP reconciles namespace attributes when appending elements to DOM
but since we previously only attached the elements after all children were attached,
the reconciliation algorithm was not able to see the root element’s attributes.
To fix this, let’s attach each element to its parent immediately after it is created.
1af6cbeb1e