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 installed, but basically, if you can compile other packages from source, you should be good to go.

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