What are the artifacts on the page when returning HTTP/1.1 404 Not Found?

Question

PHP

When the title is rendered with the following construction:
header('HTTP/1.1 404 Not Found');
on the page, in different places of the layout, some numbers with letters appear. Usually 3-4 digits followed by a small letter.
All this was cured by changing the version from HTTP / 1.1 to HTTP / 1.0
Does anyone know what it is?

Answer 1

m.geektimes.ru/post/72143
See comments.

Answer 2

It is possible that your response comes in UTF-8 with BOM (Byte Order Mark) encoding, and the browser expects another one, for example, Latin 1 (ISO 8859-1). In this case, characters appear on the page that were not in the source code. Check the source of the problematic page: at the very beginning there may be such a 
In UTF-8 without BOM, there will be no such problems.