重点：

1、在IE中下载附件之前要清空缓存。

2、中文文件名要用urlencode编码。

Header("Pragma: ");             //不加的话，IE中会提示目标主机无法访问
Header("Cache-Control: ");      //不加的话，IE中会提示目标主机无法访问
Header("content-type: $type");
Header("accept-ranges: bytes");
Header("Content-Transfer-Encoding:base64");
Header("accept-length: " . filesize($path_c));
Header("content-disposition: attachment; filename=" .urlencode($filename));  //IE中不用urlencode中文名会出现乱码
readfile($path_c);
exit;

或

header('Content-Description: File Transfer');
header('Content-Type: application/octet-stream');
header('Content-Disposition: attachment; filename='.urlencode(basename($file) ));  //IE中不用urlencode中文名会出现乱码
header('Content-Transfer-Encoding: binary');   //二进制传输
header('Expires: 0');
header('Cache-Control: must-revalidate, post-check=0, pre-check=0');    //不加的话，IE中会提示目标主机无法访问
header('Pragma: public');     //不加的话，IE中会提示目标主机无法访问
header('Content-Length: ' . filesize($file));    
ob_clean();
flush();
readfile($file);
exit;

参考链接： http://stackoverflow.com/questions/1218925/php-script-to-download-file-not-working-in-ie