急!!!php 下载execl文件 格式报错

我的程序

<?

function download($file_dir,$file_prefix,$file_name){
$file_dir = "../attachments/$file_dir/";
$file_name = urldecode($file_name);
$dl_name = $file_name;
//$file_name = $file_prefix."_".$file_name;
if (!file_exists($file_name)) { //检查文件是否存在
echo "文件找不到";
exit;
} else {
clearstatcache();
$file = fopen($file_name,"rb"); // 打开文件
// 输入文件标签
//octet-stream
header("Cache-control: private");
header("Content-type:application/vnd.ms-excel");
header("Accept-Ranges: bytes");
header("Accept-Length: ".filesize($file_name));
header("Content-Disposition: attachment; filename=" . $dl_name);
// 输出文件内容
echo fread($file,filesize($file_name));
fclose($file);
exit;
}

}
// $dir=$_GET['dir'];
// $prefix=$_GET['prefix'];
// $name=$_GET['name'];
$dir="D:\workspace\work";
$prefix="a";
$name="a.xlsx";
download($dir,$prefix,$name);
?>