As we've already discussed, the HTTP response that a server sends back to a client contains headers
that identify the type of content in the body of the response, the
server that sent the response, how many bytes are in the body, when the
response was sent, etc. PHP and Apache normally take care of the headers
for you, identifying the document as HTML, calculating the length of the
HTML page, and so on. Most web applications never need to set headers
themselves. However, if you want to send back something that's not HTML,
set the expiration time for a page, redirect the client's browser, or
generate a specific HTTP error, you'll need to use the
header( ) function.
The only catch to setting headers is that you must do so before any of the body is
generated. This means that all calls to
setcookie( ), if
you're setting cookies) must happen at the very top of your file, even
<html> tag. For
<?php header('Content-Type: text/plain'); ?> Date: today From: fred To: barney Subject: hands off! My lunchbox is mine and mine alone. Get your own, you filthy scrounger!
Attempting to set headers after the document has started results in this warning:
Warning: Cannot add header information - headers already sent
The Content-Type header identifies the type of document being
returned. Ordinarily this is
"text/html", indicating an HTML document,
but there are other useful document types. For example,
"text/plain" forces ...