为什么下载php文件失败

<?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();