There are a few ways.
One is to use a service like this: http://www.interfax.net/ You configure your site to pass them the data and they fax it to the end destination. Looks like prices range from 7 cents to 11 cents per page. (Probably calls inside the US only).
The other is to install a Fax modem into your web server and connect from PHP to server software that can drive your fax modem. This is the hard way.
