session_start();
$ads = explode("\n", file_get_contents('ads.txt'));
if (!isset($_SESSION['adcounter'])) $_SESSION['adcounter'] = array_rand($ads);
echo $ads[($_SESSION['adcounter']++)%sizeof($ads)];
<a href="http://www.free-css-templates.com" target=_blank"><img src="http://www.istoritve.com/mybanner.jpg" border="0"></a>
<a href="http://www.free-css-templates.com" target=_blank"><img src="http://www.istoritve.com/mybanner.jpg" border="0"></a>
<a href="http://www.free-css-templates.com" target=_blank"><img src="http://www.istoritve.com/mybanner.jpg" border="0"></a>