$xml_file = new XMLWriter();

$xml_file->startElement("People");

while ($list_of_people->current != NULL ){

    $xml_file->writeElement("Person"); // create a new person

    $xml_file->startElement('Person');

    $xml_file->writeAttribute('name', $list_of_people->current->name); // add their name as an attribute

    if ($list_of_people->current->children != NULL){

        while ($list_of_people->current->child_current != NULL){ // if they have children create them as well

            $xml_file->writeElement("Person");

            $list_of_people->startElement('Person');

            $xml_file->writeAttribute('name', $list_of_people->current->child_current->child_name);

            $xml_file->endElement();

            $list_of_people->current->child_current = $list_of_people->current->child_current->next;
        }
    }

    $xml_file->endElement();

    $list_of_people->current = $list_of_people->current->next;
}

<People>
 <Person name="Anna"></Person>
 <Person name="Joe">
   <Person name="Willy"></Person> // Joe has a child named Willy
 </Person>
<Person name="Rob"></Person>
</People>