<?php
$banner[1] = "http://www.banner.com/site/url.jpg";
$url[1] = "http://www.url.com";
$banner[2] = "http://www.banner.com/site/url.jpg";
$url[2] = "http://www.url.com";
$banner[3] = "http://www.banner.com/site/url.jpg";
$url[3] = "http://www.url.com";
$r = rand(1, count($banner));
echo "<a href=\"" . $url[$r] . "\" target=\"_blank\"><img src=\"" . $banner[$r] . "\" border=\"0\"></a>";
?>
just make more variables with a higher number like the first 3, for every one you wanna add