@jmikola @mgentili @alisan16 Should we create an enum type in PHP for the read preference types supported? According to http://www.php.net/manual/en/mongocursor.setreadpreference.php the supported types are: 
MongoClient::RP_PRIMARY
MongoClient::RP_PRIMARY_PREFERRED
MongoClient::RP_SECONDARY
MongoClient::RP_SECONDARY_PREFERRED
MongoClient::RP_NEAREST