MongoGridFS represents the files and chunks collections. MongoGridFS extends MongoCollection, and an instance of MongoGridFS has access to all of MongoCollection methods, which act on the files collection:
<?php
$grid = $db->getGridFS();
$grid->update(array("filename" => "foo"), $newObj); // update on the files collection
?>
Another example of manipulating metadata:
<?php
// save a file
$id = $grid->storeFile("game.tgz");
$game = $grid->findOne();
// add a downloads counter
$game->file['downloads'] = 0;
$grid->save($game->file);
// increment the counter
$grid->update(array("_id" => $id), array('$inc' => array("downloads" => 1)));
?>
You can also access the chunks collection from an instance of MongoGridFS:
<?php
$chunks = $grid->chunks; // $chunks is a normal MongoCollection
$chunks->insert(array("x" => 4));
?>
There are some methods for MongoGridFS with the same name as MongoCollection methods, that behave slightly differently. For example, MongoGridFS::remove() will remove any objects that match the criteria from the files collection and their content from the chunks collection.
To store something new in GridFS, there are a couple options. If you have a filename, you can say:
<?php
$grid->storeFile($filename, array("whatever" => "metadata", "you" => "want"));
?>
If you have a string of bytes that isn't a file, you can also store that using MongoGridFS::storeBytes():
<?php
$grid->storeBytes($bytes, array("whatever" => "metadata", "you" => "want"));
?>