Stopping the application from User_CustomValidate

Hello,I want to stop the application and display a value using any of this method : var_dump, printr inside User_CustomValidate event but it is not working.I did :
var_dump(“Hello”);
die();but the application instead of stopping continued it normal flow and didn’t show my message Hello.What can I do to make this happen?

Thanks,

Make sure you login as an user from the user table, the event will not be fired for the hard-coded administrator.

Yes I am logging using the third party service ( Facebook) it seems to be fired but not accepting to stop or display anything.