Question

Here’s an excerpt from a Wikipedia article:

In contrast to the GET request method where only a URL and headers are
sent to the server, POST requests also include a message body.

Based on that, it looks like the URL is sent separately from the header, but if that’s so, why do we use the header() method in PHP to set a URL to redirect to?

header("Location: http://google.com");

Answer 1

When you want to browse an URL from your browser you type an URL. The browser puts the url inside an HTTP REQUEST like this:

GET /path/to/resource.php?var=data1&othervar=data2 HTTP/1.1
Host: example.com
Connection: keep-alive
"empty line"

Then a webserver gives you an answer like this:

HTTP/1.0 200 OK
Date: Fri, 02 Sep 2011 14:37:36 GMT
Server: Apache
Cache-Control: private, s-maxage=0, max-age=0, must-revalidate
Content-Encoding: gzip
Vary: Accept-Encoding
Content-Length: 149
Content-Type: text/javascript; charset=utf-8
Connection: keep-alive
"empty line"
"149 bytes of Response data"

Every line like this “Header-Name: header_value\r\n” is an header.
PHP header function adds an header to the response before sending it to user’s browser.
In your example the header is:

Location: http://google.com

And it’s added just after the last header before the “empty line” (which is a line which contains only a \r\n).

POST requests are different from GET requests because you have a request body after the “empty line”):

POST /path/to/resource.php HTTP/1.1
Host: example.com
Connection: keep-alive
Content-Length: "number of bytes in the body"
"empty line"
variable=data&othervar=data2

In conclusion an HTTP request is made like this:

1. Request/response row (POST or GET followed by url and http version for request, Http version followed by response code and response string for the response) ended with \r\n
2. Request/response headers (header-name: header_value\r\n)
3. empty row (\r\n)
4. Response/request body

PS. Rows are ALWAYS closed by “\r\n” bytes (“empty lines” are made of just those two bytes).