Say we want to change the database host because we changed our hosting service provider. We happily edit parameters.yml and upload it back up to the server, but we still get the "Connection timed out" error.
The solution is clearing the cache, but without direct access to the server, doing that via FTP might take ages. In my case, a simpler solution was to delete only the app/cache/prod/appProdProjectContainer.php file, which seems to be where Symfony 2 caches its parameters.
I'd like to personally thank the debug_print_backtrace() function. If you use it, make sure to do so with the DEBUG_BACKTRACE_IGNORE_ARGS flag, or you will get an illegible chunk of text.