<?php
if (isset($_COOKIE['stamp'])) {
    if ($_COOKIE['stamp'] == md5($_COOKIE['data'] . 'SECRETKEY')) {
        echo "valid cookie\n";
    } else {
        echo "tamper temper";
    }
} else {
    $_COOKIE['data'] = '00110100011';
    $_COOKIE['stamp'] = md5($_COOKIE['data'] . 'SECRETKEY');
}
?>