Another alternative is to use an SQL server that has Windows ODBC drivers and use that to store the data, which you can then access from Microsoft Access (using ODBC) and PHP (using the built in drivers), or to use an intermediary file format that Access and PHP both understand, such as flat files or dBase databases.
If you are running PHP on a Unix box and want to talk to MS Access on a Windows box you will need Unix ODBC drivers. OpenLink Software has Unix-based ODBC drivers that can do this. There is a free pilot program where you can download an evaluation copy that doesn't expire and prices start at $675 for the commercial supported version.