Reason 1: Your PHP is higher than 5.2
Why? the mssql extension is not included in PHP Releases higher than 5.2.4, You WILL have to use Microsoft's DB Connect Extension for PHP and rewrite the script-
Reason 2: Your server has it's mssql.secure_connection option set to 0. You will need a 1 in order to connect. Check it with a phpinfo.php (Google!)
Reason 3: You just failed to insert the right Account Informations.
Reason 4: You haven't allowed Named Pipes and TCP/IP in your MSSQL Configuration.
Well just figure out what it was. And by the way, all those Informations ARE available in the web, maybe u would have to search 1-2 hours....