Safe mode has been removed from php
Neither php 5.4 or 5.5 have it.
php 5.3 has reach end of life
http://php.net/archive/2012.php#id2012-12-20-1
The point being, there is no reason for XenForo to go out of their way to support it. Suggest you find a new host if they're dependent on "safe mode"