Drill 20: Teach it — Forms and superglobals

<?php
$email = filter_input(INPUT_POST, 'email', FILTER_VALIDATE_EMAIL);
$error = null;

if ($_SERVER['REQUEST_METHOD'] === 'POST' && !$email) {
    $error = 'Enter a valid email address.';
}
?>
<form method="post">
  <label>Email <input name="email" type="email"></label>
  <button>Subscribe</button>
</form>
<?php if ($error): ?>
  <p><?= htmlspecialchars($error) ?></p>
<?php endif; ?>