I just figured out my problem hope this will help others because it took me about 3 hours of my life lol.
It's not sufficient to enable the MSSQL-extension. Take a look at the requirements for the Win32-platform:
"The extension requires the MS SQL Client Tools to be installed on the system where PHP is installed. The Client Tools can be installed from the MS SQL Server CD or by copying ntwdblib.dll from \winnt\system32 on the server to \winnt\system32 on the PHP box. Copying ntwdblib.dll will only provide access. Configuration of the client will require installation of all the tools."
[taken from http://www.php.net/manual/en/ref.mssql.php]