Let's talk about the expansion of header usage in php

header() is used to send rawheaders. See the HTTP/1.1 specification for more information onheaders.

Example 1:

<?PHP
Header("Location: http://www.php.cn"); 
exit;//在每个重定向之后都必须加上“exit",避免发生错误后，继续执行。
?>

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example 2: Disable the page from being cached in IE
So that the viewer can get the latest information every time, instead of the information in the Proxy or cache:

<?PHP
header( &#39;Expires: Fri, 4 Dec 2009 09:00:00 GMT&#39; );
header( &#39;Last-Modified: &#39; . gmdate( &#39;D, d M Y H:i:s&#39; ) . &#39; GMT&#39; );
header( &#39;Cache-Control: no-store, no-cache, must-revalidate&#39; );
header( &#39;Cache-Control: post-check=0, pre-check=0&#39;, false );
header( &#39;Pragma: no-cache&#39; ); //兼容http1.0和https
?>

CacheControl = no-cache Pragma=no-cache Expires = -1
If the web pages on the server change frequently, set Expires to -1, indicating immediate expiration. If a web page is updated at 1 am every day, you can set Expires to 1 am the next day. When the HTTP1.1 server specifies CacheControl = no-cache, the browser will not cache the web page.
Legacy HTTP 1.0 servers cannot use the Cache-Control header. So for backward compatibility with HTTP 1.0 servers, IE provides special support for HTTP using the Pragma:no-cache header. If the client communicates with the server over a secure connection and the server returns the Pragma:no-cache header in the response, Internet Explorer does not cache the response.
Note: Pragma:no-cache only prevents caching when used in a secure connection. If used in a non-secure page, the processing method is the same as Expires:-1. The page will be cached but marked as immediate. Expired.
http-equiv meta tag:
http-equiv meta can be used to mark the specified http message header in the html page. Older versions of IE may not support html meta tags, so it's best to use http message headers to disable caching.
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Example 3: Let the user's browser appear Unable to find file information.
A lot of information on the Internet says this: PHP's function header() can send a Status header to the browser,
such as header("Status: 404 Not Found"). But in fact the response returned by the browser is:

HTTP/1.x 200 OK
Date: Thu, 03 Aug 2006 07:49:11 GMT
Server: Apache/2.0.55 (Win32) PHP/5.0.5
X-Powered-By: PHP/5.0.5
Status: 404 Not Found
Content-Length: 0
Keep-Alive: timeout=15, max=98
Connection: Keep-Alive
Content-Type: text/html

After checking some information, the correct way to write it is:

header(”http/1.1 404 Not Found”);

The first part is the version of the HTTP protocol (HTTP-Version) ; The second part is the status code (Status); the third part is the reason phrase (Reason-Phrase).
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Example 4: Allow users to download files (hidden files The location)
html tag can realize ordinary file download. If you want to keep the file confidential and you cannot tell others the file link, you can use the header function to download the file.

<?php
header("Content-type: application/x-gzip"); 
header("Content-Disposition: attachment; filename=文件名/"); 
header("Content-Description: PHP Generated Data"); 
?>

Example 4: Input content before the header function
Generally speaking, html content cannot be output before the header function. Similarly, there are setcookie() and session functions. These functions require Add message header information to the output stream. If there are statements such as echo before header() is executed, when header() is encountered later, a "Warning: Cannot modify header information - headers already sent by ...." error will be reported. That is to say, there cannot be any text, blank lines, carriage returns, etc. in front of these functions, and it is best to add the exit() function after the header() function. For example, in the following incorrect writing, there is a blank line between the two php code sections:

//some code here
<?php
//这里应该是一个空行
header(”http/1.1 403 Forbidden”);
exit();
?>

原因是：PHP脚本开始执行 时,它可以同时发送http消息头部(标题)信息和主体信息. http消息头部(来自 header() 或 SetCookie() 函数)并不会立即发送,相反,它被保存到一个列表中. 这样就可以允许你修改标题信息,包括缺省的标题(例如 Content-Type 标题）.但是,一旦脚本发送了任何非标题的输出（例如,使用 HTML 或 print() 调用),那么PHP就必须先发送完所有的Header,然后终止 HTTP header.而后继续发送主体数据.从这时开始,任何添加或修改Header信息的试图都是不允许的,并会发送上述的错误消息之一。
解决办法：
修改php.ini打开缓存(output_buffering),或者在程序中使用缓存函数ob_start()，ob_end_flush()等。原理是：output_buffering被启用时,在脚本发送输出时，PHP并不发送HTTP header。相反，它将此输出通过管道（pipe）输入到动态增加的缓存中（只能在PHP 4.0中使用，它具有中央化的输出机制）。你仍然可以修改/添加header，或者设置cookie，因为header实际上并没有发送。当全部脚本终止时，PHP将自动发送HTTP header到浏览器，然后再发送输出缓冲中的内容。
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PHP 手册实例应用
1：您可以使用heder命令，强制使浏览器使用新鲜的内容（无缓存） 。
也可以给网址增加了一个唯一的编号，使其每次都读取新的内容，避免缓存。
example:

<?php
print "<img src="cs.jpg" mce_src="cs.jpg">";   //通常读取的是缓存文件
?>
<?
print "<img src="cs.jpg?".time()."" mce_src="cs.jpg?".time()."">";   //增加了唯一的编号，使浏览器重新请求
w//print "<img src="cs.jpg?".rand(100,999)."" mce_src="cs.jpg?".rand(100,999)."">"; 
?>

2: 下面是个很好的函数，将图片传送给浏览器显示。

<?php
function PE_img_by_path($PE_imgpath = "")
{
    if (file_exists($PE_imgpath)) {
        $PE_imgarray = pathinfo($PE_imgpath);
        $iconcontent = file_get_contents($PE_imgpath);
        header("Content-type: image/" . $PE_imgarray["extension"]);
        header(&#39;Content-length: &#39; . strlen($iconcontent));
        echo $iconcontent;
        die(0);
    }
    return false;
} 
?>

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