I would use PHP code widget and put in <? include "bannerphpfileurl.php" ?> in the sidebar.
Then in an external php file on your host, do this:
Code:
<?php
$advert = array();
$advert[] = '<a href=""><img src="" />advert1</a>';
$advert[] = '<a href=""><img src="" />advert2</a>';
$advert[] = '<a href=""><img src="" />advert3</a>';
shuffle($advert);
echo $advert[0];
?>
Then either use tracking on the index that tracks exits, or if the URLs to the ads are the same, make separate redirect pages on your server for each ad to keep track.
Can someone modify this to make tracking better?..
I guess you don't need the php for the banners to be external, but it makes working on multiple sites much easier..

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