<?php 
srand( microtime() * 1000000); 
$advertise = rand(1,3); 
//After the rand decided how many youve got and the first and last number e.g got 3, 1,3
switch($advertise) 
//you can add more just use the same format
{ 
case 1 : $banner="site 1 image url"; $url="site one address"; break; 
case 2 : $banner="site 2 image url"; $url="site two address"; break; 
case 3 : $banner="site 3 image url"; $url="site 3 address"; break; 
} 
$display = "<a href=\"$url\" target=\"_blank\"> "; 
$display.= "<img src=\"$banner\"  "; 
$display.= "width=\"468\" height=\"60\" border=\"0\" >"; 
$display.="</a>"; 
echo($display); 
?>