DOCUMENT_ROOT in PHP 7+, how to fix error?

Question

PHP

Warning: Use of undefined constant DOCUMENT_ROOT - assumed 'DOCUMENT_ROOT' (this will throw an Error in a future version of PHP) in
$fp = fopen($_SERVER[DOCUMENT_ROOT].'
The question is how to fix it without writing the full path.

Answer 1

Actually, this warning will be in PHP 5 as well. The DOCUMENT_ROOT constant is not defined, so PHP assumed it needed the string 'DOCUMENT_ROOT'. To avoid a warning, add quotes.

Answer 2

somewhere at the top of the index.php file must be added
in this way, I fixed the error for myself
you can protect yourself even more

if (isset($_SERVER['DOCUMENT_ROOT']))
    define('DOCUMENT_ROOT', $_SERVER['DOCUMENT_ROOT']);
else
    define('DOCUMENT_ROOT', null);