fopen warning

   $comment_helper = null;
    $comments = [];
    $fp = fopen($_SERVER['DOCUMENT_ROOT']. "1460725204439623926.txt", "a");

    do {
        if (is_null($comment_helper)) {
            $comment_helper = $i->getMediaComments(1460725204439623926);
        } else {
            $comment_helper = $i->getMediaComments(1460725204439623926, $comment_helper->getNextMaxId());
        }
        $comments = array_merge($comments, $comment_helper->getComments());
    } while (!is_null($comment_helper->getNextMaxId()));

    foreach ($comments as $comment) {
        try {
            if(strlen($comment->text) == mb_strlen($comment->text, 'utf-8')) {
                fwrite($fp, '- '.$comment->text."\n");
            }
        }
        catch (InstagramException $ie) {
            echo $ie.getCode();
            sleep(6);
        }
       fclose($fp);
    }