Php 编写文件，然后压缩文件并在提交时下载

我正在尝试创建一个文件（works），压缩一个文件（works），然后下载。但是，当我把它按我认为正确的顺序排列时，我会收到一条警告，说标题已经发送了

我的代码是：

//TOP Writes to file -this works
$handle=fopen('db_backup.sql','w+');
fwrite($handle,$end);
fclose($handle);

//Creates zip -this works
$zip   =array('db_backup.sql');
create_zip($zip,'db_backup.zip',true);
ob_clean();
flush();

//Download the zip get the headers already sent message
header('Content-Description: File Transfer');
header('Content-Type: application/octet-stream');
header('Content-Disposition: attachment; filename=db_backup.zip');
header('Content-Transfer-Encoding: binary');
header('Expires: 0');
header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
header('Pragma: public');
header('Content-Length: ' . filesize('db_backup.zip'));
readfile('db_backup.zip');
unlink($zip_file);

但是如果我把标题移到顶部，我就不会得到消息。但是拉链是空的

header('Content-Description: File Transfer');
header('Content-Type: application/octet-stream');
header('Content-Disposition: attachment; filename=db_backup.zip');
header('Content-Transfer-Encoding: binary');
header('Expires: 0');
header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
header('Pragma: public');
header('Content-Length: ' . filesize('db_backup.zip'));

//TOP Writes to file -this works
$handle=fopen('db_backup.sql','w+');
fwrite($handle,$end);
fclose($handle);

//Creates zip -this works
$zip   =array('db_backup.sql');
create_zip($zip,'db_backup.zip',true);
ob_clean();
flush();

//Download the zip get the headers already sent message

readfile('db_backup.zip');
unlink($zip_file);

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脚本的完整输出是什么，是否有任何警告？在文件的开始标记之前是否有任何字符？您是否将脚本保存为UTF-8（尝试ASCII）？当您说脚本时，这是否意味着创建了文件？