You don't need a new driver you just need to use the inbuilt PHP function mssql_connect("host", "username", "password").
Read the PHP manual for more details.
One thing to note though, I have been using PHP for a while mainly with MySQL, I got the trial of MS SQL 2000 but when I tried to connect to it failed. This is because I had a PHP distrubution that did not have MS SQL support configured. So if you try and connect to MS SQL and receive a fatal error: call to undefined function mssql_connect(), then check your PHP settings first before ripping your hair out.