Website misbehavior. Who is guilty?

Question

PHP

Hello!
There was a problem: Symbols and tags that should be inside the head are added to the markup and go to the body. What could it possibly be?
The site is assembled dynamically from header.php, body.php and footer.php
Here is the structure of the document:

And if you open the element inspector, then it looks like this:

wtf?!
UPDATE:
An absolutely(!) empty html document that is not even related to php in any way and contains only three characters: 123

Has the structure in the inspector:

How is this even possible?! Maybe the host "helps"?

Answer 1

UTF without BOM

Answer 2

and if you open ctrl + U

Answer 3

How can your "absolutely(!) empty html document" have no structure in the inspector? In any case, the browser builds the DOM tree, because Content-Type: text/html;
To see exactly the body that came to you, open it not in the inspector, but by Ctrl + U, as you have already been told.
Well, the same applies to the first question - rewrite the structures so that it is correct and does not look correct.