header is used to send a raw HTTP header. See the HTTP/1.1 specification for more information on HTTP headers.
Remember that header must be called before any actual output is sent, either by normal HTML tags, blank lines in a file, or from PHP. It is a very common error to read code with include, or require, functions, or another file access function, and have spaces or empty lines that are output before header is called. The same problem exists when using a single PHP/HTML file.
/* This will give an error. Note the output
* above, which is before the header() call */
The header string.
There are two special-case header calls. The first is a header that starts with the string "HTTP/" (case is not significant), which will be used to figure out the HTTP status code to send. For example, if you have configured Apache to use a PHP script to handle requests for missing files (using the ErrorDocument directive), you may want to make sure that your script generates the proper status code.
// This example illustrates the "HTTP/" special case
// Better alternatives in typical use cases include:
// 1. header($_SERVER["SERVER_PROTOCOL"] . " 404 Not Found");
// (to override http status messages for clients that are still using HTTP/1.0)
// 2. http_response_code(404); (to use the default message)
header("HTTP/1.1 404 Not Found");
The second special case is the "Location:" header. Not only does it send this header back to the browser, but it also returns a REDIRECT (302) status code to the browser unless the 201 or a 3xx status code has already been set.
header("Location: http://www.example.com/"); /* Redirect browser */
/* Make sure that code below does not get executed when we redirect. */
The optional replace parameter indicates whether the header should replace a previous similar header, or add a second header of the same type. By default it will replace, but if you pass in false as the second argument you can force multiple headers of the same type. For example:
header('WWW-Authenticate: NTLM', false);
Forces the HTTP response code to the specified value. Note that this parameter only has an effect if the header is not empty.
No value is returned.
Exceptions and Errors
On failure to schedule the header to be sent, header issues an E_WARNING level error.
Headers will only be accessible and output when a SAPI that supports them is in use.
You can use output buffering to get around this problem, with the overhead of all of your output to the browser being buffered in the server until you send it. You can do this by calling ob_start and ob_end_flush in your script, or setting the output_buffering configuration directive on in your php.ini or server configuration files.
The HTTP status header line will always be the first sent to the client, regardless of the actual header call being the first or not. The status may be overridden by calling header with a new status line at any time unless the HTTP headers have already been sent.
Most contemporary clients accept relative URIs as argument to Location:, but some older clients require an absolute URI including the scheme, hostname and absolute path. You can usually use $_SERVER['HTTP_HOST'], $_SERVER['PHP_SELF'] and dirname to make an absolute URI from a relative one yourself:
/* Redirect to a different page in the current directory that was requested */
$host = $_SERVER['HTTP_HOST'];
$uri = rtrim(dirname($_SERVER['PHP_SELF']), '/\\');
$extra = 'mypage.php';
Session ID is not passed with Location header even if session.use_trans_sid is enabled. It must by passed manually using SID constant.