The function is being called from your server, so you need to edit your php.ini file, and look for the following extension line:
;extension=php_mssql.dll
Remove the semi-colon, and make sure the dll file is in your extensions directory. Then restart your webserver.
(Assuming you are running a Windows based machine.)
This will enable you to run the mssql functions. I don't know if this will enable you to connect to a remote host from your server, but it will get you started with running mssql functions anyhow.