php Header文件下载

/**
* 文件下载
* @param string $path  文件所在路径
* @param string $filename 文件名
*
* $dir="F:/file/";
$filename="f.txt";
file_down($dir,$filename);
*/
function file_down($dir,$filename){
$path=$dir.$filename;
if(!file_exists($path)){
exit("文件不存在");
}
//打开文件
$file = fopen ($path, "r" );
//输入文件标签
Header ( "Content-type: application/octet-stream" );
Header ( "Accept-Ranges: bytes" );
Header ( "Accept-Length: " . filesize ($path ) );
Header ( "Content-Disposition: attachment; filename=" . $filename );
//输出文件内容
//读取文件内容并直接输出到浏览器
echo fread ( $file, filesize ( $path ) );
fclose ( $file );
exit ();
}