What I think I hear you say is you want to be able to detect if more than one IP address logs in with the same username? You want to use this to detect account sharing? Please clarify.

Detecting the IP address is very easy, create a PHP page with only this code in it:
[PHP]
<php
phpinfo();
?>
[/PHP]
Run the page in a browser, then look for the server variables such as "REMOTE_ADDR". In PHP, you access these using the $_SERVER object.
[PHP]
$ip = $_SERVER['REMOTE_ADDR'];
[/PHP]

I should add that this is not a good way to detect account sharing unless you want to make it clear to your users that they can only use their account on a single computer and if the cable/dsl company changes their dynamic IP, they will lose access to their account.

I should also point out that the opposite is true, one IP could be hundreds of people. Almost any company on the Internet has all of its employees surfing the web behind a single IP--the IP of their firewall or router. In the old days, IP used to equate to unique identity on the Internet. That is no longer true.