problem with overwritting php cookie

I am having a problem with cookies using php.

I have pages that track what affiliate sent me traffic, and what the keyword for the add is.

ID is the affiliates ID

http://www.mysite.com/index.php?ID=100&keyword=abc

I have the code below on msyite.com/index.php, and mysite.com/someotherpage.php

When a user click thru to http://www.mysite.com/index.php?ID=100&wk=abc
, then goes to
mysite.com/someotherpage.php
that has the same code below,(the same code as mysite.com/index.php) the cookies are overwritten from ID=100&wk=abc to values of 0., and  I can't capture the id when it is uploaded into a database.

<?php
 $ID_club = $_GET['ID'];
 $keyword = $_GET['keyword'];
setcookie("club_id_refer", $ID_club);
setcookie("club_id_refer", $ID_club, time()+3600);  /* expire in 1 hour */
setcookie("keyword_refer", $keyword);
setcookie("keyword_refer", $keyword, time()+3600);  /* expire in 1 hour */
 ?>

How can I make sure that this cookie is not overwritten from the page that the user first arrives at?

Thanks for the help!