Why doesn't ajax work when using define php?

Question

PHP

Good afternoon, when using the define constant, ajax stops working, while all other functions work, and ajax answers that there is no constant.

Handler The file from which ajax calls to the handler
defined('_JEXEC') or die('Restricted access');
define( '_JEXEC', 1 )

Answer 1

First, the essence of the question is not clear.
Secondly, how can Ajax notify that a constant does not exist from PHP ?
Thirdly, there are no psychics here.