PHP：使用PDO和Failover_Partner连接到MSsql

I need to connect from a Linux (LAMP) machine to an SQL server with mirroring.

I tried using the following but with no luck:

$dbh = new PDO("dblib:version=7.0;charset=UTF-8;host=10.37.25.2;dbname=myDataBaseName","$username","$pw", array("Failover_Partner" => "10.37.25.3:1433"));

I can connect to each server but when I try the above with the master (10.37.25.2) off I get the following error:

Failed to get DB handle: SQLSTATE[01002] Adaptive Server connection failed (severity 9)

Any suggestions?