This is the unified Sybase-DB style driver for PDO.
There are three implementations of this library that are known to me:

- The Sybase DB lib itself
- MSSQL DB lib
- FreeTDS DB lib

This extension will compile and register itself as 'mssql' when built against
the mssql libraries (and be named php_pdo_mssql.dll), or 'sybase' otherwise
(php_pdo_sybase.dll)

If you want to try out the free "desktop" version of SQL Server, known as the MSDE, google to obtain the appropriate download link.  Here are some short tips on getting it running:

- Download it and run it to extract it
- Open up a command prompt
- cd \MSDERelA
- setup.exe SQLSECURITY=1 SAPWD=yoursecretpassword
- cd \Program Files\Microsoft SQL Server\80\Tools\Binn
- SVRNETCN.exe
-   enable TCP    (you MUST do this if you want to access it via FreeTDS/Sybase libs)
- net start mssqlserver

Consult the MS docs for more information about all this stuff.