Description:
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Note: I added the following as a manual note first, but Thiago Henrique Pojda of php-notes rejected it with the recommendation that I file it as a documentation bug. I make my conclusions based on http://cvs.php.net/viewvc.cgi/php-src/main/SAPI.c?revision=1.232&view=markup.

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The manual is not completely precise on what PHP does (at least
according to the PHP source in CVS) with Location: headers.

This is the actual behaviour when you specify a Location: header, as
far as I could derive:
 - If in earlier header() calls(*), a status code 201 or 300 - 307 was
already specified, then that status code is retained
 - If no status code was specified, or if the status code specified
earlier is not 201 or 300 ... 307 then:
  - If the request method is GET or HEAD then 302 "Found" is set
  - Otherwise (i.e. for POST or PUT), 303 "See Also" is set.

Finally, if in the same header call you specify a custom status code,
that one is always set (no matter its value), unless changed later
again.

Examples:
 <?php
 header("Location: http://www.foobar.com/");
 ?>
and
 <?php
 header("HTTP/1.1 404 Not Found");
 header("Location: http://www.foobar.com/");
 ?>
set the status code to 302 when called with GET and to 303 when called
with POST, and

 <?php
 header("Location: http://www.foobar.com/", true, 404);
 ?>
and
 <?php
 header("Location: http://www.foobar.com/");
 header("HTTP/1.1 404 Not Found");
 ?>
set the status code to 404.

Note that combining a Location header with a status code other than
201 or 3xx does not really make much sense in HTTP <= 1.1 (hence PHP's
behaviour); as such, the last three examples are just for illustrative
purposes.

(*) either through a header("HTTP/1.1 302 Found") - style call or
through the third argument on a different header() call, i.e.
header("Content-type: text/html", true, 302); (which does not make
much sense, though)