The MSSQL extension is enabled by adding extension=php_mssql.dll to php.ini.
To get these functions to work, you have to compile PHP with --with-mssql[=DIR], where DIR is the FreeTDS install prefix. And FreeTDS should be compiled using --enable-msdblib.
MS SQL functions are aliases to Sybase functions if PHP is compiled with Sybase extension and without MS SQL extension.