<?php

$cursor = $collection->find(array(), array('news' => 1));

// See: http://www.mongodb.org/display/DOCS/Advanced+Queries#AdvancedQueries-{{snapshot%28%29}}
$cursor->snapshot();

foreach ($cursor as $document) {
    $collection->update(
        array('_id' => $document['_id']),
        array('$set' => array('sum' => array_sum($document['news']))),
        array('multiple' => false)
    );
}