Enable TLS certificate verification for AWS RDS MySQL - php

Question

I use MySQL 5.7.17 on AWS RDS.
I encountered a strange behavior and I am looking for an explanation.
In short: I try to connect over SSL, with settings that I think should cause the connection to FAIL, but it succeeds!
The following PHP code succeeds to connect to the RDS instance over SSL:
<?
$HOST = "something.amazonaws.com";
$USER = "myuser";
$PASS = "mypass";
$connectionString = "mysql:host={$HOST};charset=utf8";
$options = [ ];
$options[PDO::MYSQL_ATTR_SSL_CA] = "LITERALLY THIS TEXT. DEFINITELY NOT A CERTIFICATE!";
$options[PDO::MYSQL_ATTR_SSL_VERIFY_SERVER_CERT] = false;
$conn = new \PDO($connectionString, $USER, $PASS, $options);
$sql = "SHOW STATUS LIKE 'Ssl_cipher'";
$stmt = $conn->prepare($sql);
$stmt->execute([ ]);
$stmt->setFetchMode(\PDO::FETCH_ASSOC);
$rows = $stmt->fetchAll();
print_r($rows);
The result I get:
Array
(
[0] => Array
(
[Variable_name] => Ssl_cipher
[Value] => AES256-SHA
)
)
Also, I found three pem files in /etc/ca-certificates/rds-mysql. I thought that PHP might be going there for some reason, so I deleted them, but the SSL connection still succeeds.
Note: if I delete the line that says $options[PDO::MYSQL_ATTR_SSL_CA] = "LITERALLY THIS TEXT. DEFINITELY NOT A CERTIFICATE!"; - it does NOT connect over SSL. So it appears that this option does have some impact.
My question is: how come it succeeds?