Suhosin will not accept this because it's too easy to exploit. Instead, you need to be explicit:
require($_SERVER['DOCUMENT_ROOT'] . '/classes/SomeClass.php');
The DOCUMENT_ROOT element of the $_SERVER global array will always point to the full path of your website's root. (eg. /home/user/Websites/mywebsite.com).
Please ensure you comply with this more secure style of including files.