Description:
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http://ca2.php.net/manual/en/function.header.php contains:

header — Send a raw HTTP header

header() is used to send a raw HTTP header. See the » HTTP/1.1 specification for more information on HTTP headers.


However, sending raw HTTP headers is only one of header()'s functions. Other important functions include sending a Status-Line and setting CGI fields.

In fact, the documentation page itself shows such examples:


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.

<?php
header("HTTP/1.0 404 Not Found");
?>

For FastCGI you must use the following for a 404 response:
<?php
header("Status: 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.

<?php
header("Location: http://www.example.com/"); /* Redirect browser */

/* Make sure that code below does not get executed when we redirect. */
exit;
?>


CGI fields are documented in http://tools.ietf.org/html/rfc3875#section-6.3
Status is *not* an HTTP header - running
header("Status: 404 Not Found");
when not in CGI mode will send a raw "Status" HTTP header, which is undefined.