PHP下载大文件失败并限制下载速度的实例代码

<?php//设置文件最长执行时间set_time_limit(0);if (isset($_GET['filename']) && !empty($_GET['filename'])) {  $file_name = $_GET['filename'];  $file = __DIR__ . '/assets/' . $file_name;} else {  echo 'what are your searching for?';  exit();}if (file_exists($file) && is_file($file)) {  $filesize = filesize($file);  header('Content-Description: File Transfer');  header('Content-Type: application/octet-stream');  header('Content-Transfer-Encoding: binary');  header('Accept-Ranges: bytes');  header('Expires: 0');  header('Cache-Control: must-revalidate');  header('Pragma: public');  header('Content-Length: ' . $filesize);  header('Content-Disposition: attachment; filename=' . $file_name);  // 打开文件  $fp = fopen($file, 'rb');  // 设置指针位置  fseek($fp, 0);  // 开启缓冲区  ob_start();  // 分段读取文件  while (!feof($fp)) {    $chunk_size = 1024 * 1024 * 2; // 2MB    echo fread($fp, $chunk_size);    ob_flush(); // 刷新PHP缓冲区到Web服务器    flush(); // 刷新Web服务器缓冲区到浏览器    sleep(1); // 每1秒 下载 2 MB  }  // 关闭缓冲区  ob_end_clean();  fclose($fp);} else {  echo 'file not exists or has been removed!';}exit();