MongoDB GridFS Get ID of a file - php

Question

I am using MongoDB GridFS to store user avatars. I am using Laravel 4.2.
I am writing a function to clean up the DB of any unused avatars; an avatar is unused if it's id is not associated with any user (User model). Given the ID of an avatar I can remove the file from the DB. However I am having trouble extracting ID's of an avatar so I can compare it with those in the User model.
The data looks like this:
$avatars = DB::collection('user_avatars.files')->get();
return Response::json( $avatars[0] );
//result:
{"_id":{"$id":"542797096a8d09ac318b456b"},"extension":"jpg","usage":0,"popularity":[],"filename":"image.jpg","uploadDate":{"sec":1411880713,"usec":671000},"length":248388,"chunkSize":262144,"md5":"2c724361015c7e438d30359dd9c724a0"}
Now if I write:
return Response::json( $avatars[0]['_id'] );
//result is:
{"$id":"542797096a8d09ac318b456b"}
How would I grab 542797096a8d09ac318b456b? Anything I have tried so far does not give the ID but throws an error:
$avatars[0]['_id']->$id;
$avatars[0]['_id']['$id'];
$avatars[0]['_id']->$$id;
$avatars[0]['_id']->{$id};

Answer 1

I tried this and it works great:
avatars[0]['_id']->{'$id'};
Here is the complete function:
public function doPurgeDB()
{
$removed = array();
$avatars = DB::collection('user_avatars.files')->get();
foreach( $avatars as $avatar )
{
$avatar_id = $avatar['_id']->{'$id'}; //<<<<=========
$user = User::where('avatar_id', '=', $avatar_id);
if( !$user->count() )
{
$removed[] = $avatar;
$this->removeAvatar( $avatar_id );
}
}
return Response::json( $removed );
}
public function removeAvatar( $id )
{
$grid = DB::getGridFS('user_avatars');
return $grid->delete( new MongoId( $id ) );
}

Answer 2

Cast it to string: (string)$avatars[0]['_id'];