http://www.php.net/manual/en/intro.mssql.php
"This extension is not available anymore on Windows with PHP 5.3 or later."
So anything 5.2.x and below. However you should note that 5.2 and lower is no longer supported and won't even get security fixes.
Ideally you really should look into upgrading your 13 year old SQL server, it is not supported by Microsoft any more. I'd try and push IT on this.