composer require wechatpay/wechatpay

<?php
require_once('./vendor/autoload.php');

$mchid = '';
$key = '';
$apiclient_key = '/path/to/apiclient_key.pem';
$apiclient_cert = '/path/to/apiclient_cert.pem';

use WeChatPay\Builder;
use WeChatPay\Transformer;

$instance = Builder::factory([
    'serial'     => 'nop',
    'privateKey' => 'any',
    'certs'      => ['any' => null],
    'mchid'      => $mchid,
    'secret'     => $key,
    'merchant'   => [
        'key' => $apiclient_key,
        'cert' => $apiclient_cert,
    ]
]);

$res = $instance->v2->risk->getpublickey->post([
    'xml' => [
        'mch_id' => $mchid,
        'sign_type' => 'MD5',
    ],
    'security' => true,
    'base_uri' => 'https://fraud.mch.weixin.qq.com/',
]);
var_dump(Transformer::toArray((string)$res->getBody()));

array(5) {
  ["return_code"]=>
  string(7) "SUCCESS"
  ["return_msg"]=>
  string(2) "OK"
  ["result_code"]=>
  string(7) "SUCCESS"
  ["mch_id"]=>
  string(10) ""
  ["pub_key"]=>
  string(426) "-----BEGIN RSA PUBLIC KEY-----
MIIBCgKCAQEA43u03cMF3QEgWDOy2nhUCEu0QxUzNRVgQfghRSmweBSeuufh6keu
Qca30T+RwsiUU5tafKZFgeaUbOiQTvtBvt49pYUZKoWufl/RBqWkLtf0eOeaAzRF
cAbGPm+ZM8tHhsFBAnrKASe72gZEUGodMsS7jRG9lGQMFPoksUwLVcnXYmHkOQrm
KvF8AwkBgdVaMgyYShMys+sLjfrtcMc+SnnTt7T7aJBjsyhoRWvyF3OL+rMwwPfN
4+ugAOopBuGMCwPz2GRXLzzxoJqIhAlO/WxqlII95i0wrI9QL224HbW1C0RWrvGe
Ugr8GSWz3ywXPWUf3VIDGLwXZE8gN6liDwIDAQAB
-----END RSA PUBLIC KEY-----
"
}