This afternoon we published version 1.3.0 of the MongoDB PHP driver. Besides a number of bug fixes since RC2 and RC3, this new release also includes a new MongoClient class. This new MongoClient class serves as a replacement for the Mongo class. The old Mongo class is now deprecated and will be removed in a future release, although we are keeping it in place for now because of backwards compatibility reasons. We have already removed it mostly from the documentation, and are working to update all our other material as well.
The main change that comes with the MongoClient class is that it now has acknowledged writes on by default (a "safe mode"). This option determines wether or not the client waits for a confirmation from the server when a write has happened. He includes a bit of code showing how to: turn it off, keep it on or using replica set acknowledged writes. You can also set it on a per-query basis.