To connect to a shard cluster, specify the address of one or more mongos instances in the connection string. Multiple servers may be delimited by a comma.
// Using one server as the seed list
$m = new MongoClient("mongodb://mongos1.example.com:27017");
// Using multiple servers as the seed list
$m = new MongoClient("mongodb://mongos1.example.com:27017,mongos2.example.com:27017"));
Regardless of whether each shard is a stand-alone mongod server or a full replica set, the driver's connection process is the same. All database communication will be routed through mongos.
For more information on sharding with MongoDB, see the » sharding documentation.