php XMLreader:编辑元素属性&值

如何编辑XML元素的属性或值?

<element>
  <apple type="fruit">red</apple>
</element>

XMLreader :

        $xml= new XMLReader();
        $xml->open($file);
        while($xml->read()){
            if($xml->nodeType == XMLReader::ELEMENT) {
                if($xml->getAttribute('type') == "fruit") {
                       //change attr. and values to:
                       //apple = cabbage, fruit = vegetable, red = white
                }
            }               
        }