In place where I use require_once('something.php');
I have strange char in html and when I check page on validator:
Validation Output: 1 Error
Error Line 1, Column 1: character "" not allowed in prolog
This only happens when I'm using UTF-8. Earlier I have files in ANSI and it was ok.
Yes I'm changing meta and save all files in UTF-8 I can move code to section body but this is strange for me.
This is most probably caused by the UTF-8 BOM (Byte Order Mark). Open any file in some HEX viewer / editor and check the first 3 bytes in that file.
UTF-8 BOM in Windows-1250 encoding looks like this: 
. Or 
in ISO-8859-1. That's EF BB BF
in hexadecimal.
Just save your files as UTF-8 without BOM. For example Notepad++ editor has both options under Format
menu: