<?php
$article = "hello <a href='http://www.someotherwebsite.com'>testlink</a> everyone" ;
$parts = explode(" ", $article);
$randomamount = count($parts);
$hit = rand(0, $randomamount);
if(isset($_SESSION['last'])) {
    // already seen?ok
    if($hit == 0) {
        $hit=$hit+1;
    } else {
        $hit=$hit-1;
    }
}
$_SESSION['last']=$hit;
echo $parts[$hit];
?>