Description:
------------
Trying to connect to an AWS RDS MySQL Instance with PDO using an encrypted SSL connection using the combined CA bundle provided here: https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/UsingWithRDS.SSL.html#UsingWithRDS.SSL.IntermediateCertificates results in the following error:

Fatal error: Uncaught PDOException: PDO::__construct(): SSL operation failed with code 1. OpenSSL Error messages:
error:1416F086:SSL routines:tls_process_server_certificate:certificate verify failed in /ssl/test.php:4

I tested this with the official PHP Docker images and using a native Debian Buster installation.

I tried to find out when this broke and pinned it to version 7.3.7. In 7.3.6 everything worked fine. 

In the changelog I found this change:
Fixed bug #78079 (openssl_encrypt_ccm.phpt fails with OpenSSL 1.1.1c).

This change was also included in 7.2.20 and I was able to confirm the issue there as well. With 7.2.19 it works fine.

In the Docker images, PHP 7.3.6 and 7.2.19 use OpenSSL version 1.1.0k, 7.3.7 and 7.2.20 use 1.1.1c.
The native Buster installation was using PHP 7.3.11 with OpenSSL 1.1.1d.

Using the Alpine Docker build or an Ubuntu installation works fine. So I guess this is related to the Debian Buster environment.

The test script tries to connect to an RDS instance. However, I think you can use any MySQL instance as the certificate validation fails locally. I do not assume that the CA bundle is invalid, as it works on several other environments.

Test script:
---------------
# Dockerfile

FROM php:7.3.7-cli
RUN docker-php-ext-install pdo_mysql


# test.php
<?php

$pdo = new PDO('mysql:host=foobar.abc-central-1.rds.amazonaws.com', 'admin', 'egal', [
    PDO::MYSQL_ATTR_SSL_CA => './rds-combined-ca-bundle.pem',
    PDO::MYSQL_ATTR_SSL_VERIFY_SERVER_CERT => true,
]);