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();