I needed to migrate an old website which still relied on an MSSQL connection not supported in our new server I just built. Luckily someone else has already figured this one out (http://ubuntuforums.org/showthread.php?t=350269&page=3)

Here's the gist of it (pertaining to php5):

aptitude install php5-sybase

This installs the generic support for mssql (FYI, mssql is based on sybase) but is missing a few mssql specific functions for php.

pear install --nodeps MDB2_Driver_mssql

This command installs those extra functions and everything seems to work. Cheers!