XML dom replace childs

Here is xml file

<stuff>
  <foo>
    <bar></bar>
      <value/>
    <pub></pub>
  </foo>
  <foo>
    <bar></bar>
    <pub></pub>
  </foo>
  <foo>
    <bar></bar>
    <pub></pub>
  </foo>
</stuff>

Then i trying to use code

$new=$dom>createTextNode('newnodeName');

$elements = $dom->getElementsByTagName('foo');
foreach($elements as $node){
    foreach($node->childNodes as $child) {
        $child->parentNode->replaceChild($new, $child); 
    }
}

1. Why this code not replace each $child by $new value


2. REturn output in string format not in XML?


3. How to change root TagName "stuff" to "team" ?