// links array
$links = array(...);
 
// get users visited links to an array
$visited_links = explode('|', $_SESSION['visited_links']);
 
// remove visited links from links array
foreach($visited_links as $visited_link) {
unset($links[array_search($visited_link, $links)]);
}
 
// get a random link from unvisited links
$link = $links[rand(0, count($links)-1)];
 
// add the selected link to visited array
$visited_links[] = $link;
 
// save visited links to user session as | separated string
$_SESSION['visited_links'] = implode('|', $visited_links);