SSL证书有效期监测，实现相对简单，具体代码如下

<?php
/**
 * TIT Theme Customizer
 *
 * @package TIT
 */

/*
// 每天检查一次 SSL 状态
if ( ! wp_next_scheduled( 'check_ssl_status_daily' ) ) {
    wp_schedule_event( time(), 'daily', 'check_ssl_status_daily' );
}
*/

add_action( 'check_ssl_status_daily', 'check_ssl_status_function' );

function check_ssl_status_function() {
    $host = parse_url(home_url(), PHP_URL_HOST);
    $port = 443;
    
    $contextOptions = [
        "ssl" => [
            "capture_peer_cert" => true,
            "verify_peer" => false,
            "verify_peer_name" => false,
        ],
    ];

    $context = stream_context_create($contextOptions);
    $client = @stream_socket_client("ssl://{$host}:{$port}", $errno, $errstr, 30, STREAM_CLIENT_CONNECT, $context);

    if (!$client) {
        send_ssl_notification_email("SSL 检测失败：无法连接到服务器。错误：$errstr");
        return;
    }

    $params = stream_context_get_params($client);
    $cert = openssl_x509_parse($params["options"]["ssl"]["peer_certificate"]);

    if (!$cert || empty($cert['validTo_time_t'])) {
        send_ssl_notification_email("SSL 检测失败：无法解析证书。");
        return;
    }

    $expiryTimestamp = $cert['validTo_time_t'];
    $expiryDate = date('Y-m-d', $expiryTimestamp);
    $remainingDays = ceil(($expiryTimestamp - time()) / 86400);

    $message = "🔐 当前 SSL 证书有效期至：". $expiryDate ."（剩余 $remainingDays 天）";

    // 可自定义天数阈值
    if ($remainingDays <= 7) {
        send_ssl_notification_email("警告：网站 SSL 证书将于 $expiryDate 到期，仅剩 $remainingDays 天，请尽快续期。");
    }

    send_ssl_notification_email($message);
}

function send_ssl_notification_email($message) {
    $admin_email = get_option('admin_email');
    $extra_email = 'info@titstudio.com';
    $recipients = [$admin_email, $extra_email];

    $subject = '【SSL 警告】网站证书问题提醒';
    $body = "您好，\n\n您的网站（" . home_url() . "）的 SSL 证书状态如下：\n\n$message\n\n请及时处理以免影响访问。\n\n-- TIT网站监控";
    
    wp_mail($recipients, $subject, $body);
}

// 手动触发测试 SSL 邮件：访问 https://yourdomain.com/?ssl_check_test=1
add_action('init', function () {
    if (isset($_GET['ssl_check_test']) && $_GET['ssl_check_test'] == '1') {
        check_ssl_status_function();
        wp_die('SSL 检查已触发，请检查邮箱'); // 输出提示防止空白页
    }
});