I think rsmith had it right. Leave the MS Access DB on your PC (I assume it is on the local network protected by a firewall) and run the PHP server on the public network (Outside the firewall). Configure the firewall so that the only device that can access anything on the private network is the web server.
On the local PC side you will need to set up ODBC. See http://www.w3schools.com/php/php_db_odbc.asp
On the PHP server you will need to setup a connection and logic for CRUD (Create, Retrieve, Update, Delete). There are a number of sites around to show you this but here are a few:
http://www.phpfreaks.com/tutorials/61/0.php
http://www.web-max.ca/PHP/odbc_1.php
Not to mention the definitive PHP Site:
http://us3.php.net/manual/en/ref.uodbc.php