Analyse d'un exemple de contrôle de session PHP

<?php
session_start();
define(&#39;u&#39;,&#39;a&#39;);
define(&#39;p&#39;,&#39;1&#39;);
if (isset($_GET[&#39;r&#39;]) && $_GET[&#39;r&#39;]== 1) {
  unset($_COOKIE[&#39;username&#39;]);
  unset($_COOKIE[&#39;password&#39;]);
  unset($_SESSION[&#39;valid_login&#39;]);
}
if (isset($_POST[&#39;username&#39;]) && isset($_POST[&#39;password&#39;])) {
  $username = $_POST[&#39;username&#39;];
  $password = $_POST[&#39;password&#39;];
  if ($username == u && $password == p) {
    $_SESSION[&#39;valid_login&#39;] = 1;
    setcookie(&#39;username&#39;, $username);
    setcookie(&#39;password&#39;, $password);
  } else {
    echo &#39;incorrect u or p&#39;;
  }
}
?>
<?php
if (isset($_SESSION[&#39;valid_login&#39;])) {
  printf(&#39;welcom, %s&#39;, $_COOKIE[&#39;username&#39;]);
} 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>