<?phpsession_start();define('u','a');define('p','1');if (isset($_GET['r']) && $_GET['r']== 1) {  unset($_COOKIE['username']);  unset($_COOKIE['password']);  unset($_SESSION['valid_login']);}if (isset($_POST['username']) && isset($_POST['password'])) {  $username = $_POST['username'];  $password = $_POST['password'];  if ($username == u && $password == p) {    $_SESSION['valid_login'] = 1;    setcookie('username', $username);    setcookie('password', $password);  } else {    echo 'incorrect u or p';  }}?><?phpif (isset($_SESSION['valid_login'])) {  printf('welcom, %s', $_COOKIE['username']);} else {?><form action="" method="post">  <input type="text" name="username" value="" />  <input type="password" name="password" value="" />  <input type="submit" /></form><?php}?><br /><a href="cookie_session.php" title="">refresh</a><br /><a href="cookie_session.php?r=1" title="">clear</a>