This provider will default to the write concern and read preference
options set on the MongoCollection instance (or inherited from MongoDB or
MongoClient). Using an unacknowledged write concern (< 1) may make the
return values of delete() and save() unreliable. Reading from secondaries
may make contain() and fetch() unreliable.
See also:
http://www.php.net/manual/en/mongo.readpreferences.php
See also:
http://www.php.net/manual/en/mongo.writeconcerns.php