Web Programmer Needed For a \'Cookie / Session Variable\' Job (PHP/Cookies)

What I want to do is change a line of text on a website depending on which landing page someone comes from.

For example:

Type in www.mywebsite.co.uk in browser (display normal phone numbers on the website 0800 111 111)

Coming from landing page 1 ---> www.mywebsite.co.uk (all the places that display the phone number will be automatically changed to 0800 999 999)

Coming from landing page 2 ---> www.mywebsite.co.uk (all the places that display the phone number will be automatically changed to 0800 888 888)

Coming from landing page 3 ---> www.mywebsite.co.uk (all the places that display the phone number will be automatically changed to 0800 777 777)

Hope this is clear enough, I think there are two ways of going this; one would be to use cookies (serverside or client side), another would be that each landing page populates a session variable (for PHP or .Net) and then use the session variable where ever needed.

I\'d like it so that if someone goes through a landing page it not only changes the phone numbers they see for that session, but for at least a month in the future when they revisit even by typing in URL. I\'m guessing that means using cookies.

I\'ve been told it\'s a reasonably simple bit of code, but I can not program to save my life. The winner bidder will be expected to test and ensure that the code works, of course.