You will most likely need to use the SQLSRV extension rather than the MSSQL one (they are both for MS SQL Server but MSSQL extension was discontinued some time ago and removed from PHP 5.3 and onward http://us2.php.net/manual/en/intro.mssql.php
See here: http://www.php.net/manual/en/sqlsrv.requirements.php
1. You will need to grab the Microsoft SQL Server 2012 Native Client x86 install and the SQLSRV 3.0 install from the sqlsrv link above.
2. Extract the SQLSRV to somewhere convenient and copy the "php_sqlsrv_54_ts.dll" and "php_pdo_sqlsrv_54_ts.dll" files to your xampp/php/ext folder
3. Add the proper extension lines to your php.ini "extension=php_sqlsrv_54_ts.dll" and "extension=php_pdo_sqlsrv_54_ts.dll"
4. Restart apache and you should be good to go.
Note: if you have existing code that uses the mssql functions, you will need to port it to the sqlsrv ones.