Mongo autoincrement field

gistfile1.php

<?php
$func = new MongoCode(
    'function insertDocument(doc, targetCollection) {
        targetCollection = db[targetCollection];
        var cursor = targetCollection.find( {}, { _id: 1 } ).sort( { _id: -1 } ).limit(1);
        var seq = cursor.hasNext() ? cursor.next()._id + 1 : 1;
        doc._id = NumberInt(seq);
        var results = targetCollection.insert(doc);
        return doc._id;
    }'
);
$response = $db->execute($func, array($data, $collectionName));
echo $response['retval']['value']; //insert id