首先下载一个Acmephp在git上可以下载到

下载地址：https://github.com/acmephp/acmephp

我这里演示的是最新版本2.1.0

接下来直接上代码

<?php

require_once __DIR__.'/vendor/autoload.php';

use AcmePhpSslCertificateRequest;
use AcmePhpSslKeyPair;
use AcmePhpSslParserKeyParser;
use AcmePhpCoreAcmeClient;
use AcmePhpCoreAcmeServerDirectory;
use AcmePhpCoreExceptionAcmeException;

// 配置你的ACME服务器
$directoryUrl = 'https://acme-v02.api.letsencrypt.org/directory';
$email = 'your-email@example.com';
$serverDirectory = new AcmeServerDirectory($directoryUrl, $email);

// 使用你持有的域名来构建你的证书申请
$domain = 'example.com';
$keyParser = new KeyParser();
$keyPair = $keyParser->parse(file_get_contents('/path/to/private-key.pem'));
$csr = new CertificateRequest($keyPair, [$domain]);

// 初始化ACME客户端
$acmeClient = new AcmeClient($serverDirectory);

try {
    // 获取新的SSL证书
    $certificate = $acmeClient->requestCertificate($csr, $domain);
} catch (AcmeException $e) {
    echo 'ACME异常：'.$e->getMessage();
}

// 将证书以PEM格式写入文件
file_put_contents('/path/to/certificate.pem', $certificate->getCertificate());