in the link to the desktop site, include a query string in the url. this can be used to prevent the browser check code from functioning
Link:
<a href="http://fullsite.com/index.php?mobilecheck=no">view full site</a>
browser detection:
<?php
if(isset($_GET['mobilecheck'])) $_SESSION['mobilecheck'] = $_GET['mobilecheck'];
if(!isset($_SESSION['mobilecheck']) || $_SESSION['mobilecheck'] != "no) {
<brower detection code>
}
?>