Sorry, but your problem does not imply a bug in PHP itself.  For a
list of more appropriate places to ask for help using PHP, please
visit http://www.php.net/support.php as this bug system is not the
appropriate forum for asking support questions.  Due to the volume
of reports we can not explain in detail here why your report is not
a bug.  The support channels will be able to provide an explanation
for you.

Thank you for your interest in PHP.

For me, the feof() also works badly.

   while($max && !feof($src))
   {
      $l=$max>0x100000 ? 0x100000 : $max;
      $s=fread($src,$l);
      if($s===false || $s==='') return 1;
      if(fwrite($dst,$s)!==strlen($s)) return 2;
      if(strlen($s)!==$l) break;
      $max-=$l;
   }

In this code the "$str===false || $str===''" should be not reachable without file problem but it is reachable on Linux. WHY??? 

If the function (feof) works correctly (IMO it doesn't work correctly due to manual definition) - HOW TO CHECK WHY I CANNOT READ FILE? Because of permisions,  or file delete OR real end of file? 

PHP 7.4, Linux. I believe there is real problem in feof implementation in PHP.

Some people don't understand how eof works.
`eof` is the end of file (stream) byte.
To understand that the end of the file (stream) has been reached while reading, this byte must be `read (fread ($ res, 1);` or `fgetc ($ res));`
Even when you move the pointer with `fseek ($ res, $ offset)`, you won't determine the end of the file until you read the eof byte.

    public function isNextEof($res,int $size=1):bool
    {
        $tell=ftell($res);
        $check=$this->fread($res,$size);
        $eof=feof($res);
        fseek($res,$tell);
        return $eof;
    }

Be sure to ask the error to find out why file functions return false or empty strings.
https://www.php.net/manual/en/function.socket-last-error.php
https://www.php.net/manual/en/function.error-get-last.php  

Please place this text in coments
https://www.php.net/manual/en/function.feof.php 
(in my case, the provider's IP is for some reason in the black list)

Minor edits to my previous comment.
`eof` it  the  end file byte;
Editing the code
public function isNextEof($res,int $size=1):bool
    {
        $tell=ftell($res);
        //$check=fread($res,$size);
        fseek($res,$size-1,SEEK_CUR);
        $check=fgetc($res);
        $eof=feof($res);
        fseek($res,$tell);
        return $eof;
    }