Add a number in a config file

Hey guys, I was wondering if someone could help me out with a plugin I'm making. So I want to be able to store a number in a config file, then add onto that number. For example:
When a player votes, it will get the player's file, find the number in the file, and add 1 to the number that is in the file.
Does anyone know how i can do this? I know how to do math in php, but when I try to use it with a config file it doesn't work.

PHP:

$config->set('number', (intval($config->get('number') + 1));
$config->save();

Thanks