Can I get the IP address from a visitor?

Hi! This is probably the wrong forum. Please redirect me if im out of place. I would like to find out what IP address my visitor uses when visiting a php script page on my domain. This page would contain a form in which the visitor would state his/her message. My goal is to create purchase-orders written on unique files, and to separate them my idea was to baptize files with the IP addresses, which would ive each visitor an unique purchase-order... Any hints to a newbie? Cheers, Lars