为什么不；t此PHP下载脚本工作

以下是我编写的一个简单脚本，用于限制用户一次只能下载一个文件（IE，如果他们正在下载一份文件，则在取消当前下载或下载完成之前，他们无法下载另一份文件）。

ignore_user_abort(true);
$local_file = $_GET['filename'];
$download_file = explode("/", $local_file);
$download_file = $download_file[count($download_file) -1];
// set the download rate limit (value is in kilobytes per second
$download_rate = 100;
if(file_exists($local_file) && is_file($local_file)) {
    $ip = visitor_ip();
    if(!are_downloading($ip)) {
        header('Cache-control: private');
        header('Content-Type: application/octet-stream');
        header('Content-Length: '.filesize($local_file));
        header('Content-Disposition: filename='.$download_file);
        flush();
        $file = fopen($local_file, "r");
        log_downloader($ip);
        while(!feof($file)) {
            if (!connection_aborted()) {
            // send the current file part to the browser
            print fread($file, round($download_rate * 1024));
            // flush the content to the browser
            flush();
            // sleep one second
            sleep(1);
        } else {
            break;
            }
        }
        clear_downloader($ip);      
        fclose($file);
    } else {
        die('<span style="color:#DDDDDD">Due to server limitations you may only download one file at a time. Please cancel or wait for your current download to finish before trying again. Click <a href="/simfiles">here</a> to return.</span>');
    }
} else {
    die('Error: The file '.$local_file.' does not exist!');
}
function visitor_ip() { 
    if(isset($_SERVER['HTTP_X_FORWARDED_FOR']))
        $TheIp=$_SERVER['HTTP_X_FORWARDED_FOR'];
    else $TheIp=$_SERVER['REMOTE_ADDR'];
    return trim($TheIp);
}
function are_downloading($ip) {
    $query = "select * from downloaders where ip_addr='$ip'";
    $result = mysql_query($query);
    $num_rows = mysql_num_rows($result);
    return $num_rows > 0; 
}
function log_downloader($ip) {
    $query = "insert into downloaders (ip_addr) values ('$ip')";
    $result = mysql_query($query);
}
function clear_downloader($ip) {
    $query = "delete from downloaders where ip_addr='$ip'";
    $result = mysql_query($query);
}

当我测试它时，它工作得很好，但对很多人来说，他们的IP永远不会从数据库中清除——即使他们已经下载/取消了一个文件。为什么IP不被删除？