You may want to consider using a configuration file with a define for the path. Our rule of thumb is to avoid relative paths whenever possible. Thus, you would have something like this in your configuration file:
When ever you reference the needed class, you would use the corresponding require statement:
require_once(SYSTEM_CLASS_DIR . '/specificclassfile.php');
This method will also save a lot of headaches if you move from one environment to another as all you need to do is change your DEFINE value.