How to connect MS sql database and php using xamp
You don't have the MS SQL Drivers installed. You can check this with phpinfo();

On Linux you need mssql.so or sybase.so With debian its apt-get install php5-sybase

For windows take a look here: http://msdn.microsoft.com/en-US/library/cc793139%28v=SQL.90%29.aspx

Drivers need to be configured for PHP to find the function mssql_...

You could also look at PDO DB classes as they can connect to any DBS, you need the drivers installed.