php 实现文件下载 避免直接打开

function download($filename){
if(empty($filename))return false;
if (!file_exists($filename)){
header("Content-type: text/html; charset=utf-8");
echo "File not found!";
exit;
} else {
$file = fopen($filename,"r");
Header("Content-type: application/octet-stream");
Header("Accept-Ranges: bytes");
Header("Accept-Length: ".filesize($filename));
Header("Content-Disposition: attachment; filename=".basename($filename));
echo fread($file, filesize($filename));
fclose($file);
}
}