php文件下载显示找不到文件怎么办

<?php
$filename = $_GET['filename'];
//从浏览器获取到的字符的编码是UTF-8，我们要用这个函数转换成GBK才能才本地找到这个文件
$filename = mb_convert_encoding($filename,'GBK','UTF-8');
echo $filename ."<br>";
if( empty($filename)){
 echo'<script> alert("非法连接 !"); location.replace ("index.php") </script>'; exit();
}
if (!file_exists($filename)) { //检查文件是否存在
 echo "文件找不到";
 exit;
} else {
 $file = fopen($filename,"r"); // 打开文件
 // 输入文件标签
 Header("Content-type: application/octet-stream");
 Header("Accept-Ranges: bytes");
 Header("Accept-Length: ".filesize($filename));
 Header("Content-Disposition: attachment; filename=" . $filename);
 // 输出文件内容
 echo fread($file,filesize($filename));
 fclose($file);
 exit();
}
?>