(PECL mongo >=1.3.0)
Set the read preference for this connection
public bool MongoClient::setReadPreference ( string $read_preference [, array $tags ] )
Parameters:
read_preference
The read preference mode: MongoClient::RP_PRIMARY, MongoClient::RP_PRIMARY_PREFERRED, MongoClient::RP_SECONDARY, MongoClient::RP_SECONDARY_PREFERRED, or MongoClient::RP_NEAREST.