PHPのヘッダーの用途は何ですか

header('Location:'.$url);  //Location和":"之间无空格

header('content-type:text/html;charset=utf-8');

header('HTTP/1.1 404 Not Found');

header('Refresh: 10; url=http://www.baidu.com/');  //10s后跳转

header("Expires: Mon, 26 Jul 1997 05:00:00 GMT");
header("Last-Modified: " . gmdate("D, d M Y H:i:s") . "GMT");
header("Cache-Control: no-cache, must-revalidate");
header("Pragma: no-cache");

header('HTTP/1.1 401 Unauthorized');
header('WWW-Authenticate: Basic realm="Top Secret"');

header('Content-Type: application/octet-stream'); //设置内容类型
header('Content-Disposition: attachment; filename="example.zip"'); //设置MIME用户作为附件
header('Content-Transfer-Encoding: binary'); //设置传输方式
header('Content-Length: '.filesize('example.zip')); //设置内容长度