为什么MailChimp不起作用？我的代码在这里

Question

index.php代码：

<form  role="form" action="form.php" method="post">
          <input type="email" id="emailSub" placeholder="Enter your Email Adress" size="30">
          <button type="button" id="submit" class="btn">
</form>

form.php代码：

<?php
include_once 'MailChimp.php';

$alertclass = 'alert-warning';
$msg = '';
$email = '';

if (isset($POST['submit'])) {
    if (empty($_POST['emailSub'])) {
        $msg = 'Please enter a email address.';
    } else {
        $email = filter_var($_POST['emailSub'], FILTER_SANITIZE_EMAIL);
        $mc = new MailChimp('MY-API-KEY');
        $result = $mc->call('lists/subscribe', array(
                'id'                => 'MY-LIST-ID',
                'email'             => array('email'=>$email),
                'double_optin'      => true,
                'update_existing'   => false,
                'replace_interests' => false,
                'send_welcome'      => false
            )
        );
        if (!empty($result['euid'])) {
            $msg = 'Thanks, please check your mailbox and confirm the subscription.';
            $alertclass = 'alert-success';
        } else {
            if (isset($result['status'])) {
                switch ($result['code']) {
                    case 214:
                    $msg = 'You\'re already a member of this list.';
                    break;
                    default:
                    $msg = 'An unknown error occurred.';
                    $alertclass = 'alert-error';
                    break;
                }
            }

        }
        echo $msg;
    }
}
?>

我将它上传到支持5.4 PHP版本的服务器，因此它应该可以工作，但它不会。什么也没发生，没有添加订阅我的mailchimp列表，甚至没有订阅消息。而且我不知道为什么。有什么想法吗？