PDO_DBLIB is a driver that implements the PHP Data Objects (PDO) interface to enable access from PHP to Microsoft SQL Server and Sybase databases through the FreeTDS libary.
This extension is not available anymore on Windows with PHP 5.3 or later.
On Windows, you should use SqlSrv, an alternative driver for MS SQL is available from Microsoft: » http://msdn.microsoft.com/en-us/sqlserver/ff657782.aspx .
If it is not possible to use SqlSrv, you can use the PDO_ODBC driver to connect to Microsoft SQL Server and Sybase databases, as the native Windows DB-LIB is ancient, thread un-safe and no longer supported by Microsoft.