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

There is nothing in the HTTP headers to indicate which ISP a user is coming from, so the answer is no, there is no PHP builtin function which will tell you this. You'd have to use some sort of service or library which maps IPs to networks/ISPs.