'db' => [
	'class' => 'yii\db\Connection',
	'dsn' => 'mysql:host=REMOTE.HOST.MYSQL;dbname=remote_db_name',
	'username' => 'remote_db_user',
	'password' => 'remote_db_password',
	'charset' => 'utf8',
	'attributes' => [
		PDO::MYSQL_ATTR_SSL_KEY => dirname(dirname(__DIR__)) . '/common/config/ssl/client-key.pem',
		PDO::MYSQL_ATTR_SSL_CERT => dirname(dirname(__DIR__)) . '/common/config/ssl/client-cert.pem',                
		PDO::MYSQL_ATTR_SSL_CA => dirname(dirname(__DIR__)) . '/common/config/ssl/server-ca.pem',                
		],
],

This is assuming that the certs are within the common folder of an Yii2 advanced template project.

It’s probably also a good idea to chmod the pem files to 600 and the ssl directory to 700 so that only the user

Where are the pem files?

/var/lib/mysql/*.pem