return $this->belongsTo('bar'); should not work on any system. Some filesystems are case-insensitive, but class names are always case-sensitive and methods are always case-insensitive.

the question is that in my system the class name loaded are case-insensitive, and it makes a different behavior on my machines, there is a php version when it becomes case insensitive by default in any system (i think 5.5) but does not seen to be true, i will learn more about it and update it ASAP

Case sensitivity is defined by the os when the drive is formatted - so in OSX Disk Utility you get a choice of "Mac OS Extended (Journaled)" which is the default for OSX users or "Mac OS Extended (Case-sensitive, Journaled)" so if you are developing for a case sensitive system you will either need to ensure that your code accounts for this or you could reformat your drive as a case sensitive and reinstall the OS.