Many have made very successful sites whose sole purpose is to echo the client IP address. A lot of tech heads have their favorites such as http://ipchicken.com or http://whatismyip.com, to name a couple. This is done simply by writing this script:
<?php echo $_SERVER['REMOTE_ADDR']; ?>
For example, your IP address is:
So, if you ever find yourself asking "What is my IP?" or "How can I get the IP of a user accessing my site?" then this is the way to do it in PHP.