WordPress プラグインにサブスクリプション機能を追加する方法

global $wpdb;
$wpdb->query("CREATE TABLE IF NOT EXISTS {$wpdb->prefix}subscribers (
    id INT AUTO_INCREMENT PRIMARY KEY,
    email VARCHAR(255) NOT NULL
)");

function wpse_subscription_form() {
    if (isset($_POST['email']) && !empty($_POST['email'])) {
        global $wpdb;
        $email = $_POST['email'];

        $wpdb->insert("{$wpdb->prefix}subscribers", array(
            'email' => $email,
        ));

        echo '订阅成功！';
    }
    else {
        echo '<form action="" method="POST">
            <label for="email">请输入您的邮箱地址：</label>
            <input type="email" name="email" id="email" required />
            <input type="submit" value="订阅" />
        </form>';
    }
}

function wpse_subscribers_page() {
    global $wpdb;

    if (isset($_GET['action']) && $_GET['action'] === 'delete') {
        $id = $_GET['id'];
        $wpdb->delete("{$wpdb->prefix}subscribers", array('id' => $id));
    }

    $subscribers = $wpdb->get_results("SELECT * FROM {$wpdb->prefix}subscribers");

    if (!empty($subscribers)) {
        echo '<table>
            <tr>
                <th>ID</th>
                <th>Email</th>
                <th>操作</th>
            </tr>';

        foreach ($subscribers as $subscriber) {
            echo '<tr>
                <td>' . $subscriber->id . '</td>
                <td>' . $subscriber->email . '</td>
                <td><a href="?page=subscribers&action=delete&id=' . $subscriber->id . '">删除</a></td>
            </tr>';
        }

        echo '</table>';
    }
    else {
        echo '没有订阅用户！';
    }
}