<?php
$html[0] = '<a href="link here"><img src="banner here" /></a>';
$html[1] = '<a href="link here"><img src="banner here" /></a>';
$html[2] = '<a href="link here"><img src="banner here" /></a>';
$html[3] = '<a href="link here"><img src="banner here" /></a>';
$html[4] = '<a href="link here"><img src="banner here" /></a>';
$r = rand(0, count($html));
echo $html[$r];
?>