print url of refering domain

Hi all, Reading the title of this message you might think this is an easy one to answer, but it isnt as straight forward as it might first seem. I have a number of domain names that are set up on my dns server but they have no site associated with them so any visitors to these domains just end up at the home page of the nameserver with http://domain.colo.magmom.net in the address bar. I would like to be able to display the refering domain name on the page along with a small text message (under construction) etc, to save me having to create sites for each domain. Is this possible? Examples please, I am a complete novice at PHP :) Many thanks - Mark Williams