Derick Rethans has a new post to his site detailing some of the "read" preferences that you can customize in the latest versions of the MongoClient functionality in the MongoDB PHP extension for replica sets and sharing setups.
Read Preferences are a new Replica Set and Sharding feature implemented by most MongoDB drivers that are supported by 10gen. This functionality requires MongoDB 2.2. In short, Read Preferences allow you to configure from which nodes you prefer the driver reads data from. In a Replica Set environment it is the driver that does the selection of the preferred node, and in a Sharded environment it is the mongos process that routes queries according to the defined Read Preferences.
He starts with a look at the read preference types (like "primary", "secondary" and "nearest") how the connection manager works to handle each type. He includes some code samples showing how to configure your MongoClient connections to use these various types of preferences. He also introduces the concept of "tags" for the replica set - aliases to make them a bit easier to identify when making a connection and how to define them in the connection string.