SQLSRV is not exactly a "driver": it's a completely different PHP extension to access SQL Server databases and it has its own syntax and features. It's a Windows-only library developed and maintained by Microsoft and it's not related to the built-in SQL Server extension described here.
(Whatever, it's quite feature-rich and it's easy to learn.)
These functions allow you to access MS SQL Server database.
This extension is not available anymore on Windows with PHP 5.3 or later.
SQLSRV, an alternative driver for MS SQL is available from Microsoft: » http://msdn.microsoft.com/en-us/sqlserver/ff657782.aspx.