If you do need to use an intermediary file format, OpenLink have now released Virtuoso (a virtual database engine) for NT, Linux and other unix platforms.
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. On this point Tim Hayes from OpenLink software writes: