// 创建 stream context
$context = stream_context_create([
 'ssl' => [
  'capture_peer_cert' => true,
  'capture_peer_cert_chain' => true,
 ],
]);
 
$resource = stream_socket_client("ssl://$domain:$port", $errno, $errstr, 30, STREAM_CLIENT_CONNECT, $context);
$cert = stream_context_get_params($resource);
 
$ssl = $cert['options']['ssl'];
$resource = $ssl['peer_certificate'];
 
// 网站证书中只有公钥，通过 openssl_pkey_get_details 导出公钥
 
$ret = [
 'crt' => '',
 'pub' => '',
];
 
$pkey = openssl_pkey_get_public($resource);
$ret['pub'] = openssl_pkey_get_details($pkey)['key'];
 
openssl_x509_export($resource, $pem);
$ret['crt'] = $pem;
 
foreach ($ssl['peer_certificate_chain'] as $resource)
{
 openssl_x509_export($resource, $pem);
 $ret['crt'] .= "\n" . $pem;
}
 
// 保存 $ret['crt'] 为 domain.crt
// 保存 $ret['pub'] 为 domain.pub
 
return $ret;