Apparently, there's no package for MS-SQL support in Debian or Ubuntu. Why this is, is not very known, in particular since there is MS-SQL support in PHP4 and PHP5.
I found the following steps somewhere on the internet.

The following was tested on a Ubuntu 6.10 Edgy, but will probably work on others as well.
Also, whenever I refer to php5, it could well be replaced with php4, which I've tested as well.

First of, make sure the necessary build tools are installed:
# apt-get install build-essential debhelper

It's possible more tools are needed, depending on which ones have already been install  ...