Untitled

<?php
          session_start();
          class TOKEN {
            public static function generate() {
              return $_SESSION['token'] = base64_encode(openssl_random_pseudo_bytes(15));
            }
            public static function check($token) {
              if (isset($_SESSION['token']) && $token === $_SESSION['token']) {
                unset($_SESSION['token']);
                return true;
              }
              return false;
            }
          }
        ?>
        <?php
          $display_form = FALSE;
          if (isset($_POST['submit'])) {
            $username = $_POST['username'];
            $userpass = $_POST['userpass'];

            if (strlen($username) < 4) {
              $error_name = 'required';
              $display_form = true;
              $validation_error = true;
            }
            if (strlen($userpass) < 8) {
              $error_pass = 'required';
              $display_form = true;
              $validation_error = true;
            }
            if (!$validation_error) {
              if (TOKEN::check($_POST['token'])) {
                echo 'process form';
              } else {
                echo 'invalid security token';
              }
            }
          } else {
            $display_form = TRUE;
          }
        ?>
        <!DOCTYPE html>
        <html lang="en">
        <head>
          <meta charset="UTF-8">
          <title>Title</title>
        </head>
        <body>
        <?php
          if ($display_form == true) {
        ?>
        <form method="post" action="<?php echo htmlspecialchars($_SERVER['REQUEST_URI']); ?>">
          <input type="hidden" name="token" value="<?php echo TOKEN::generate(); ?>">
          <input type="text" name="username" id="" placeholder="username">
          <?php echo $error_name; ?>
          <br>
          <input type="password" name="userpass" id="" placeholder="Password">
          <?php echo $error_pass; ?>
          <br>
          <input type="submit" name="submit" value="Sign in">
        </form>
        </body>
        </html>
        <?php
         }
        ?>