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

This seems to be what you're looking for, it will attempt to return the full hostname if possible:

http://us3.php.net/gethostbyaddr

Answer 2

I think you need to use some third party service (Possibly a web service) to lookup the IP and find the service provider.

Answer 3

<?php
$isp = geoip_isp_by_name('www.example.com');
if ($isp) {
    echo 'This host IP is from ISP: ' . $isp;
}
?>

(PECL geoip >= 1.0.2)

geoip_isp_by_name — Get the Internet Service Provider (ISP) name

http://php.net/manual/ru/function.geoip-isp-by-name.php