<?
$link[0]="link.php?id=a";
$link[1]="link.php?id=b";
$link[2]="link.php?id=c";
$link[3]="link.php?id=d";
$link[4]="link.php?id=e";
$link[5]="link.php?id=f";
shuffle($link);
?>

<a href="<? $link[0];?>" target="_blank">