The IP number is the number that identifies a computer connected to a network, for example Internet. The Ip number could be static or dynamic. The static IP remains unchanged each time that the computer connects to the network while the dynamic IP could change each time there is a connection.
In php language the way to know the number IP of a user is simple. You must create a page with .php extension and insert only a line of code, the following line:
<? echo $REMOTE_ADDR; ?>
This line prints on screen the IP number of the user that is visiting the web-page.
For example, your IP address is:
38.107.191.99