fixed

    $comment_helper = null;
    $comments = [];
    $file = "./1460725204439623926.txt";
    do {
        if (is_null($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) {
         $file_content = $comment->text;
         file_put_contents($file, $file_content."\n", FILE_APPEND);
    }