"In 'Acquiring data from the physical world' we examined how to
use a microcontroller to turn physical events (such as reading a
switch) into data and send that data to a Linux box via the serial
line. With this capability you can do things like counting people
passing through a gate or recording how the temperature changes
over time in a factory process. You can then send the data to a Web
page. In this article, we'll do it in reverse--from a Web form,
instruct a microcontroller to perform some actions.
"You can create a Web form on an Apache/Linux server, process
the data with a PHP script, then push the control instructions down
to the microcontroller over the serial line..."