The MSSQL extension is available on Win32 systems only. You can use the Sybase extension to connect to MSSQL databases from other platforms.
These functions allow you to access MS SQL Server database. 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.
The MSSQL extension is enabled by adding extension=php_mssql.dll to php.ini.