i did this a long time ago using the odbc method. you will need to enable the odbc dll in your php.ini file.
if you don't have the dll file, download the non-installer version of php and you will find it in there.
http://www.php.net/manual/en/function.odbc-connect.php
there is all the odbc functions. pretty straight forward .. nothing really new to learn, .. just use it like you were using mysql_<whatever>
you will be doing things a little differently, but not much. you don't have access to some of the cool mysql features like mysql_insert_id()
now i don't think i've ever tried, but you may want to look into the mssql functions too. i'm not sure if it is the same type of connections as MS Access, but it may be worth looking into. same thing as the odbc dll, .. comment it out, make sure you have it, then try it.
here is the php support for mssql.
http://www.php.net/manual/en/function.mssql-connect.php
this is just from my experience, .. definately look into idx's suggestion too. i know odbc is slower than most other connections out there so take that into consideration too.