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

A quick alternative. (This website allows up to 50 calls per minute.)

$json=file_get_contents("https://extreme-ip-lookup.com/json/$ip");
extract(json_decode($json,true));
echo "ISP: $isp ($city, $region, $country)
";

API details at the bottom of the page.