How to check for 404 error via PHP or Biitrix?

Question

PHP

The products page has a 404 page if the item is not found.
How to check if 404 opened or not?
And for example, if 404 opened - display on the page echo('404");

Answer 1

if a 404 page opened, then the server code has already checked for the requested element, did not find it, and has already displayed a 404 page.