mysql_error()没有显示错误

我正在尝试调试我的代码,但mysql_error()没有显示任何内容.我知道有些不对劲,因为我写的时候

or die("ERROR");

它显示ERROR.所以问题必须是那一行代码.当我写作

or die(mysql_error());

它显示空白.这是我认为有错误的行的代码:

while ($rows = mysql_fetch_array($sql6) or die(mysql_error())) {

现在这里是完整的代码块:

$sql6 = mysql_query("SELECT * FROM replies WHERE thread_id = $thread_id");
    $numRows = mysql_num_rows($sql6);
    $replies = '';
    if ($numRows < 1) {
        $replies =  "There are no replies yet, you can make the first!";
    } else {
        while ($rows = mysql_fetch_array($sql6) or die(mysql_error())) {
            $reply_content = $rows['5'];
            $reply_username = $rows['7'];
            $reply_date = $rows['8'];
            $reply_author_id = $rows['4'];

            $sql9 = mysql_query("SELECT * FROM users WHERE id = '$reply_author_id'");
            $numRows = mysql_num_rows($sql9); 
            if ($numRows < 1) {
                while ($rows = mysql_fetch_array($sql9)) {
                    $reply_user_fn = $rows['first_name'];
                    $reply_user_ln = $rows['last_name'];
                    $reply_user_id = $rows['id'];
                    $reply_user_pp = $rows['profile_pic'];
                    $reply_user_lvl = $rows['user_level'];
                    $reply_user_threads = $rows['threads'];
                    $reply_user_email = $rows['email'];

                    $replies .= '';
                    $replies .= '

';
                    $replies .= 'by: ' . $reply_username . ' - ' . $reply_date . '
';
                    $replies .= $reply_content;
                    $replies .= '
';
                    $replies .= '
';
                    $replies .= ''. $reply_username .'
';
                    $replies .= '' . $reply_user_fn.' ' .$reply_user_ln . '
';
                    $replies .= 'Threads: ' . $reply_user_threads . ' 
Level: '. $reply_user_lvl .'
Sign up date: ' . $reply_user_email/*PUT SIGNUP DATE*/ .'';
                    $replies .= '';
                    $replies .= '';
                    }
                }
            }
        }

我做错了什么,为什么PHP不显示mysql错误？谢谢