All you need to do is rename one of the php.ini-* files to php.ini and configure it as you would like (such as enabling the MSSQL extension).
Note that the MSSQL extension probably requires a DLL in order to work. All of the support libraries needed by extensions come with PHP - they are those DLL files in the root of the PHP folder. For PHP to find them, you have to add the PHP directory to your system's PATH environment variable.