How are you generating the result set? You may want to store the result querystring in the session so you can return to it.
So on the results page you could add this to the top:
<?php
@session_start();
$_SESSION['lastResultsLink'] = $_SERVER['REQUEST_URI'];
?>
Then you can use the session variable as your link back to the most recent results page.
<a href="<?php echo(isset($_SESSION['lastResultsLink'])?$_SESSION['lastResultsLink']:"resultspage.php"); ?>">back to results</a>