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.
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You need to make sure that you are using php threadsafe version and only 32-bits.
ALWAYS PERFORM ANTIVIRUS CHECKS!!