How to connect to MySQL with SSL connection
Finally got it figured out. It's "driver options", not "PDO attributes".

  $databases = array (
    'default' =&gt; 
     array (
       'default' =&gt; 
        array (
          'database' =&gt; 'drupaldb',
          'username' =&gt; 'drupaluser',
          'password' =&gt; 'password',
          'host' =&gt; '192.168.10.100',
          'port' =&gt; '3306',
          'driver' =&gt; 'mysql',
          'prefix' =&gt; '',
          'driver options' =&gt; array(
              PDO::MYSQL_ATTR_SSL_KEY =&gt; '/path/client-key.pem',
              PDO::MYSQL_ATTR_SSL_CERT =&gt; '/path/client-cert.pem',
              PDO::MYSQL_ATTR_SSL_CA =&gt; '/path/ca-cert.pem',
           ),
       ),
    ),
  );