Stateless component in PHP

php-component.php

<?php

function SubscribeButton ($subscribed, $currentUserId, $userId, $className = null) {
  if (!isset($userId) || !ctype_digit($userId) || $userId === $currentUserId) {
    return "";
  }

  $subscribeButtonClasses = "subscribe-button";

  if (!is_null($className)) {
    $subscribeButtonClasses .= $className;
  }
  
  ?>
  <form method="POST" data-subscribed="<?php echo strval($subscribed); ?>" action="/subscribe">
    <button type="submit" class="<?php echo $subscribeButtonClasses; ?>">
      <?php echo $subscribed ? "Subscribed" : "Subscribe"; ?>
    </button>
    <input type="hidden" name="user-id" value="<?php echo strval($userId); ?>">
  </form>
  <?php
}

?>