这个下载 为什么出错？？？

<?php
$file_name="mm.jpg";
if(!file_exists($file_name)){
echo "文件不存在";
return;
}
$fp=fopen($file_name,"r");
$file_size=filesize($file_name);
header("content-type:application/octet-stream");
header("Accept-Ranges:bytes");
header("Accept-length:$file-size");
header("Content-Disposition:attachment;filename=".$file_name);
$buffer=1024;
$file_count=0;
while(!feof($fp)&&($file_size-$file_count>0)){
$file_data=fread($fp,$buffer);
$file_count+=$buffer;
echo $file_data;
}
fclose($fp);
?>