下载前不能有输出

//下面是输出下载;

header ( "Cache-Control: max-age=0" );

header ( "Content-Description: File Transfer" );

header ( ‘Content-disposition: attachment; filename=‘ . basename ( $filename ) ); // 文件名

header ( "Content-Type: application/zip" ); // zip格式的

header ( "Content-Transfer-Encoding: binary" ); // 告诉浏览器，这是二进制文件

header ( ‘Content-Length: ‘ . filesize ( $filename ) ); // 告诉浏览器，文件大小

ob_clean();

flush();

echo readfile ( $filename );//输出文件;