Detailed explanation of the reasons why header location page jump fails

http header information
Header information has many functions, the most important ones are the following:
1. Jump
When the browser receives the header After Location: xxxx in the information, it will automatically jump to the URL address pointed to by xxxx. This is a bit similar to writing a jump with js. But this jump is only known by the browser, and users cannot see it regardless of whether there is anything in the content.
Example: header("Location: http://www.php.cn/");
2. Specify the content of the web page
The same XML file, if the header information If you specify: Content-type: application/xml, the browser will parse it according to the XML file format. However, if the header information is: Content-type: text/xml, the browser will parse it as stored text. (Browsers do not parse files according to extensions)
Example: header('Content-type: application/pdf');
3. Attachment
I don’t know if you have noticed it , sometimes when downloading something from some websites, after clicking the download link, the browser opens the attachment as a web page, and the contents displayed are all garbled characters. This problem is also related to the header information. Sometimes the browser determines whether to open or save based on Content-type, so sometimes it will make a wrong judgment (mainly because the website designer forgets to write Content-type). In fact, there is another way to specify that the content is an attachment and needs to be saved. This is: Content-Disposition: attachment; filename="xxxxx"
Example: header('Content-Disposition: attachment; filename="downloaded.pdf "');
//Open the file and output
readfile('original.pdf');
HTTP protocol status code
1××　Reserved
2××"Indicates that the request was successful Receiving
3×× The customer needs to further refine the request to complete the request
4×× Customer error
5×× Server error

Syntax:

header(string,replace,http_response_code)

Parameter
string: required. Specifies the header string to be sent.
replace: optional. Indicates whether this header replaces the previous header, or adds a second header. Default is true (replacement). false (allow multiple headers of the same type).
http_response_code: Optional. Forces the HTTP response code to the specified value. (Available for PHP 4 and above)
Example 1:
Redirect web page

   <?php 
      header("Location: http://www.php.cn/");
      exit;
   ?>

Example 2:
Force users to get the latest information every time they visit this page, and Instead of using the cache that exists on the client side.

   <?php  
       //告诉浏览器此页面的过期时间(用格林威治时间表示)，只要是已经过去的日期即可。  
       header("Expires:Mon,26 Jul 1970 05:00:00 GMT");  
       //告诉浏览器此页面的最后更新日期(用格林威治时间表示)也就是当天,目的就是强迫浏览器获取最新资料  
       header("Last-Modified:".gmdate("D,d M Y H:i:s")."GMT");  
       //告诉客户端浏览器不使用缓存  
       header("Cache-Control:    no-cache,    must-revalidate");  
       //参数（与以前的服务器兼容）,即兼容HTTP1.0协议  
       header("Pragma:    no-cache"); 
  ?>

Example 3:
Output status value to the browser, mainly used for access control

   <?php  
       header(&#39;HTTP/1.1 401 Unauthorized&#39;);  
       header(&#39;status: 401 Unauthorized&#39;);  
   ?>

Example 4:
To restrict a user from accessing this page, you can Set the status to 404, as shown below, so that the browser will display that the page does not exist

   <?php  
       header(&#39;HTTP/1.1    404    Not Found&#39;);  
       header("status:    404    Not Found"); //这种写法错误
   ?>

Example 5:
The location of the hidden file
html tag < A href='http://hostname/*.mp3'> can achieve 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.

header("Content-type: audio/mpeg");
header("Content-Disposition: attachment; filename=filenale");
header("Content-Description: PHP3 Generated Data");

Example 6:
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 need to be in the output stream. Add message header information. 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:

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

原因：
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 HTTPheader.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),将output_buffering=0修改成output_buffering=4096
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HTTPheader。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PHP4.0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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