How do I retrieve the visitor's ISP through PHP?

Question

How do I find out the ISP provider of a person viewing a PHP page?

Is it possible to use PHP to track or reveal it?

If I use something like the following:

Answer 1

GeoIP will help you with this: http://www.maxmind.com/app/locate_my_ip

There is a php library for accessing geoip data: http://www.maxmind.com/app/php

Attention though, you need to put the geoip db on your machine in order to make it work, all instructions are there :)