said by johnnyboyct :

In php you can do exec »us.php.net/function.exec

said by Gwellin :

...but sending them to a simple webpage can just as easily give them their IP address to copy and paste. Just look here: »/whois

said by JAAulde :

Remembering that this gives the publicly addressable IP of the device which requested the page and that it may not be the same as the actual IP address of the computer the user is using. (NAT/NAPT/etc setups, some proxies, etc will cause this to vary)

said by Gwellin :

Ya, running a local script from a browser is not going to happen, but sending them to a simple webpage can just as easily give them their IP address to copy and paste. Just look here: »/whois

said by Gwellin :

In a PHP file (on the server) you would use the following line of code:
<?php echo $_SERVER['REMOTE_ADDR']; ?>

said by tekmunki :

Javascript would be a little more secure than executing arbitrary code.

Here's an example:
»forums.sun.com/thread.jspa?threa···=3587440

said by cowboyro :

No properly configured webhost will allow you to run system scripts. Now if you run your own webserver and properly configure your security it's a different story. I do it on my Apache/PHP/Win2003 but it's only for few predefined commands and the system requires authentication in the first place