The simplest way would be to use the mssql function libraries, e.g. mssql_pconnect():
(The list at the left of that page shows you the other members of this function family, and will provide an overview of how to use them).
That would be the crude "get you up and running" approach. All of the reader comments on the PHP site will be of great help in understanding how to use SQL Server with PHP. These native functions do not usually form part of a default instalation of PHP, so you may have to compile them in yourself, or obtain the relevant RPM/Deb package/wahtever. They are the fastest way of gtting data in and out of SQL Server DBs, but they are probably a bit out of place in the object-oriented world of PHP5. It depend how longterm this work will be. The function libraries are the quick-and-dirty (very quick, in fact, you'll be up there with .NET applications in sheer speed of running). A more abstracted route such as DB::PEAR may be a better bet for longterm maintainability and portability. I've only ever had to strip data out of legacy SQL Server DBs on an as-needed basis, so I've tended to go the quick and dirty route, I must admit. You will probably need something more permanent. There aren't a lot of longterm SQL Server bods on these forums (the PHP ones, I mean), I don't think.
THere are a good many tutorials about going the ODBC approach to interrogating SQL Server 2000 using PHP, such as this one at Builder:
There won't be a lot about PHP5-specific ways of gaining access, yet, but hey, you could end up being one of the guys that writes one of them ;).
Best of luck,