I found with MS-SQL Server 2008 on Windows Server 2008 R2 that I needed to use the MSSQL Server name. The servers hostname or IP address would not work ( with or without port, and/or SQL instance name, etc.) After trying several combinations, this is what I found worked in my configuration:
$dbh = new PDO("sqlsrv:Server=foo-sql,1433;Database=mydb", $user , $pass);
The above worked with and without port (,1433). I also noticed adding the instance name (MSSQLSERVER) after the server name with a slash (\), as in "Server=foo-sql\MSSQLSERVER" caused a failure to connect.