Here you can find the Microsoft SQL extension DLL for php.
I have built these from the PHP source code from their website and using the SQL2000 SDK that I found on a original CD long time ago. I have personally not tested these files, but there might be a chance that you must find a newer version of ntwdblib.dll depending on your SQL server, the DLL provided should work with SQL Server 2008 R2.
You need to make sure that you are using php threadsafe version and only 32-bits.
ALWAYS PERFORM ANTIVIRUS CHECKS!!