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($_GET[&#39;filename&#39;]) && !empty($_GET[&#39;filename&#39;])) {
    $file_name = $_GET[&#39;filename&#39;];
    $file = __DIR__ . &#39;/assets/&#39; . $file_name;
} else {
    echo &#39;what are your searching for?&#39;;
    exit();
}
if (file_exists($file) && is_file($file)) {
    $filesize = filesize($file);
    header(&#39;Content-Description: File Transfer&#39;);
    header(&#39;Content-Type: application/octet-stream&#39;);
    header(&#39;Content-Transfer-Encoding: binary&#39;);
    header(&#39;Accept-Ranges: bytes&#39;);
    header(&#39;Expires: 0&#39;);
    header(&#39;Cache-Control: must-revalidate&#39;);
    header(&#39;Pragma: public&#39;);
    header(&#39;Content-Length: &#39; . $filesize);
    header(&#39;Content-Disposition: attachment; filename=&#39; . $file_name);
    // 打开文件
    $fp = fopen($file, &#39;rb&#39;);
    // 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 &#39;file not exists or has been removed!&#39;;
}
exit();