<?php
$r[] = '<a href="http://www.site.com" title="title" target="_blank"><img border="0" src="http://www.site.com/img1.gif" alt="title" /></a>';
$r[] = '<a href="http://www.site2.com" title="title" target="_blank"><img border="0" src="http://www.site.com/img2.gif" alt="title" /></a>';
$r[] = '<a href="http://www.site3.com" title="title" target="_blank"><img border="0" src="http://www.site.com/img3.gif" alt="title" /></a>';
$r[] = '<a href="http://www.site4.com" title="title" target="_blank"><img border="0" src="http://www.site.com/img3.gif" alt="title" /></a>';
$r[] = '<a href="http://www.site5.com" title="title" target="_blank"><img border="0" src="http://www.site.com/img3.gif" alt="title" /></a>';
shuffle($r);
echo $r[0];
?>