Hi,
Sage Line 100 (and, AFAIK Line 50) automatically installs an ODBC driver when you install the software, and configures a DSN to the Sage data.
You can install the ODBC driver on any system without installing Sage -- it's on the install disk in the ODBC folder.
For starters, install and configure the ODBC driver on your Web server. Obviously you'll need to use a valid Sage username and password when connecting via PHP.
If you've managed to use the ODBC commands to connect to Access, then you shouldn't have any problems with Sage.
There are some oddities with the Sage ODBC though: firstly, the ODBC driver is read-only (though I believe there is a third-party driver which allows writing to the Sage database, but I wouldn't recommend it).
Secondly, I've never been able to get PHP/ODBC (or ASP/ODBC) to work with Sage unless I've disabled anonymous access. It seems that despite having a valid username/password configured for the ODBC connection, the user accessing the PHP pages also needs permission to access the Sage data directly. If he/she doesn't have access, the OBDC link returns a username/password error and the ODBC link dies (and won't work again at all until IIS is restarted).
I've created several applications (in both ASP and PHP) which pull data from Sage, and getting the permissions right has always been a nightmare.
Hope this is of some help.