使用 php 下载文件，而不是处理大文件?

I'm using php to download files, rather than the file itself opening in a new window. It seems to work ok for smaller files, but does not work for large files (I need this to work on very large files). Here's the code I have to download the file:

function downloadFile($file) {   
    if (file_exists($file)) {         
        //download file
        header('Content-Description: File Transfer');
        header('Content-Type: application/octet-stream');
        header('Content-Disposition: attachment; filename='.basename($file));
        header('Content-Transfer-Encoding: binary');
        header('Expires: 0');
        header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
        header('Pragma: public');
        header('Content-Length: '.filesize($file));
        ob_clean();
        flush();
        readfile($file);
        exit;   
    };    
};

但是当我尝试下载一个大文件(例如 265mb)时，浏览器告诉我它找不到该文件?这些文件肯定在服务器上，并且该脚本适用于较小的文件.有没有办法下载类似于我已有的大文件?