My suggestion: don't do it. Run a mysql server on the PC in question, have your PHP app as the front end for the cashiers, and then if you want MS Access's reports feature, just have Access connect to the mysql database with ODBC.
Really I think this would be a piece of cake. It's true like Tony said that you can continue to use the same tables/backend and that's probably the route I'd recommend. You'll need to install MySQL's ODBC drivers on any machine that will be linking to the MySQL database.
After that create a DSN and then access the tables through that from within Access. You may want to add code later to relink the tables every time the software loads using DSN-less tables. This way the database can run on a machine that doesn't have a DSN configured.
I do recommend that you go with either MySQL or SQL Server Express as opposed to an MS Access backend but I'm not going to take the time to elaborate on why.
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