The header() function sends a raw HTTP header to a client.
header()函数的作用是：发送一个原始 HTTP 标头[Http Header]到客户端。
It is important to notice that header() must be called before any actual output is sent (In PHP 4 and later, you can use output buffering to solve this problem):
我们必须谨记，header()必须在任何结果被发送之前请求（在PHP4 及以上版本里，你可以使用输出缓冲[output buffer]来解决这个问题）
<html><?php
// This results in an error.
// The output above is before the header()
callheader('Location: http://www.example.com/');?>
Syntax
语法
header(string,replace,http_response_code)
Parameter参数 Description描述
string Required. Specifies the header string to send
必要参数。指定需要发送的header字符串
replace Optional. Indicates whether the header should replace previous or add a second header. Default is TRUE (will replace). FALSE (allows multiple headers of the same type)
可选参数。指明是否需要替代先前的header或重新添加第二个header
http_response_code Optional. Forces the HTTP response code to the specified value (available in PHP 4.3 and higher)
可选参数。使HTTP返回指定值的代码（仅在PHP4.3以上版本中支持）
Tips and Notes
注意点
Note: Since PHP 4.4 this function prevents more than one header to be sent at once. This is a protection against header injection attacks.
注意：从PHP4.4版本开始，这个函数将不允许一次发送多个header[标头]。这将保护header免受攻击。
Example 1
案例1
Prevent page caching:
禁用页面缓存
<?php
// Date in the past
header("Expires: Mon, 26 Jul 1997 05:00:00 GMT");
header("Cache-Control: no-cache");
header("Pragma: no-cache");?>
<html><body>
......
Note: There are options that users may set to change the browser's default caching settings. By sending the headers above, you should override any of those settings and force the browser to not cache!
注意：这里列出了多个header供用户选择，由此，用户可以改变浏览器默认的缓设置；通过发送上述的header，你可以忽略所有的设置，并强制禁用浏览器的缓存设置。
Example 2
案例2
Let the user be prompted to save a generated PDF file (Content-Disposition header is used to supply a recommended filename and force the browser to display the save dialog box):
让用户快速保存一个产生的PDF文件（Content-Disposition header的作用是：提供一个推荐使用的文件名并使浏览器显示“保存对话框[save dialog box]”），具体如下：
<?php
header("Content-type:application/pdf");
// It will be called downloaded.pdf
header("Content-Disposition:attachment;filename='downloaded.pdf'");
// The PDF source is in original.pdfreadfile("original.pdf");?>
<html><body>
......
Note: There is a bug in Microsoft IE 5.5 that prevents this from working. The bug can be resolved by upgrading to Service Pack 2 or later.
注意：在MS IE 5.5中存在一个bug，他会阻止上述代码的运行；你可以升级到Service Pack 2或更高版本来解决这个问题。
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