php会话控制实例有哪些？

<?php
session_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', $password);
 } else {
  echo 'incorrect u or p';
 }
}
?>
<?php
if (isset($_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>