php怎么实现代理下载文件

$url = isset($_GET['url'])? $_GET['url'] : '';

if($url){
    $content = file_get_contents($url);
    if($content){
        header('cache-control:public');
        header('content-type:application/octet-stream');
        header('content-disposition:attachment; filename='.basename($url));
        echo $content;
    }
}