$db->sql_server_info() uses something like this to cache the database version in phpBB's cache.

$cache->put('mysqli_version', $this->sql_server_version);

$cache->get('mysqli_version')

This will generally only work if you only have one database of the type mysqli, which basically breaks the whole object oriented approach of the dbal.

The cache key should probably be changed to something more unique, like

md5($dbms . $this->server . $this->dbname . $port);