The header () function sends the original HTTP header to the client.
It is important to realize that the header () function must be called before any actual output is sent (in PHP 4 and later versions, you can use the output cache to solve this problem):
<Html>
<? Php Tutorial
// Result error
// Output already exists before header () is called
Header ('Location: http://www.example.com /');
?> Syntax
Header (string, replace, http_response_code) parameter description
String is required. Specifies the header string to be sent.
Replace is optional. Indicates whether the header replaces the previous header or adds the second header.
The default value is true (replace ). False (multiple headers of the same type are allowed ).
Http_response_code is optional. Forces the HTTP response code to the specified value. (PHP 4 and later versions are available)
Tips and comments
Note: After PHP 4.4, this function prevents multiple headers from being sent at a time. This is a protection against header injection attacks.
Example
Example 1
<? 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: You may set some options to change the default cache settings of the browser. By sending the above header, you can overwrite any of these settings and force the browser not to cache them!
Example 2
You are prompted to save a generated PDF file (the Content-Disposition header is used to provide a recommended file name and force the browser to display the save dialog box ):
<? Php
Header ("Content-type: application/pdf ");
// The file will be called downloaded.pdf
Header ("Content-Disposition: attachment?filename='downloaded= '");
// The PDF source is in original.pdf
Readfile ("original.pdf ");
?>
<Html>
<Body>
...
...
<? Php
// Result error
// Output already exists before header () is called
Header ('Location: http://www.111cn.net /');
?> Syntax
Header (string, replace, http_response_code)
This function prevents multiple headers from being sent at a time.
<? Php
// Date in the past
Header ("expires: mon, 26 jul 1997 05:00:00 gmt ");
Header ("cache-control: no-cache ");
Header ("pragma: no-cache ");
?>
You are prompted to save a generated pdf file (the content-disposition header is used to provide a recommended file name and force the browser to display the save dialog box ):
When the time exceeds the specified time, the webpage content is invalid. The format is as follows:
Expires = "expires:" http-date
For example: header ("expires: fri, 31 oct 2003 18:00:00 gmt"); indicates on Friday, January 1, October 31, 2003.
Header ("expires: wed, 30 jun 2004 09:00:00 gmt"); indicates Wednesday, June 30, 2004.
Last-modified = "last-modified" ":" http-date
For example, header ("last-modified: wed, 01 jan 2003 12:00:00 gmt"); last Modification time:, January 1, January 1, 2003
Cache control
No-cache allows the server to transmit data to remote users without being saved.
No-store prevents poor versions or sensitive information from being retained.
Must-revalidate indicates that information must be re-evaluated. If the temporarily saved data (cached) is invalid, the server is connected to the user.
For more details, see: http://www.111cn.net/phper/php-function/36348.htm
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