Here's a stackoverflow link showing you how to use this BulkUpdate feature properly and format the updates you add to the class.
http://stackoverflow.com/questions/24753464/mongo-mass-update-to-lower-case
eg.
<?php
$batch->add(
array(
"q" => array( '_id' => $doc['_id'] ),
"u" => array(
'$set' => array(
'UserName' => strtolower($doc['UserName'])
)
)
)
);
?>