$ip = getenv ("REMOTE_ADDR");
if (substr_count($ip,":") > 0
&& substr_count($ip,".") == 0) {
echo 'You are using .'href="http://www.dnsstuff.com/tools/whois.ch?ip='
. $ip . '"> IPv6 ';
} else {
echo "You are using IPv4";
}
?>

Via PHP snipplet for detecting users with IPv6