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

EDIT: This method no longer works since the website it hits now blocks automatic queries (and previously this method violated the website's terms of use). There are several other good [legal!] answers below (including my alternative this this one.)

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You can get all those things from the following PHP codings.,

(.*?)<\/th>(.*?)<\/td>/s',$url,$output,PREG_SET_ORDER);
  $isp=$output[1][2];
  $city=$output[9][2];
  $state=$output[8][2];
  $zipcode=$output[12][2];
  $country=$output[7][2];
?>

  

    
ISP :

    
City :

    
State :

    
Zipcode :

    
Country :