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Working With HTTP Requests
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In order to get the most out of CodeIgniter, you need to have a basic understanding of how HTTP requests
and responses work. Since this is what you work with while developing web applications, understanding the
concepts behind HTTP is a **must** for all developers that want to be successful.

The first part of this chapter gives an overview. After the concepts are out of the way, we will discuss
how to work with the requests and responses within CodeIgniter.

What is HTTP?
=============

HTTP is simply a text-based convention that allows two machines to talk to each other. When a browser
requests a page, it asks the server if it can get the page. The server then prepares the page and sends
a response back to the browser that asked for it. That's pretty much it. Obviously, there are some complexities
that you can use, but the basics are really pretty simple.

HTTP is the term used to describe that exchange convention. It stands for HyperText Transfer Protocol. Your goal when
you develop web applications is to always understand what the browser is requesting, and be able to
respond appropriately.

The Request
-----------
Whenever a client (a web browser, smartphone app, etc) makes a request, it sends a small text message
to the server and waits for a response.

The request would look something like this::

	GET / HTTP/1.1
	Host codeigniter.com
	Accept: text/html
	User-Agent: Chrome/46.0.2490.80

This message displays all of the information necessary to know what the client is requesting. It tells the
method for the request (GET, POST, DELETE, etc), and the version of HTTP it supports.

The request also includes a number of optional request headers that can contain a wide variety of
information such as what languages the client wants the content displayed as, the types of formats the
client accepts, and much more. Wikipedia has an article that lists `all header fields
<https://en.wikipedia.org/wiki/List_of_HTTP_header_fields>`_ if you want to look it over.

The Response
------------

Once the server receives the request, your application will take that information and generate some output.
The server will bundle your output as part of its response to the client. This is also represented as
a simple text message that looks something like this::

	HTTP/1.1 200 OK
	Server: nginx/1.8.0
	Date: Thu, 05 Nov 2015 05:33:22 GMT
	Content-Type: text/html; charset=UTF-8

	<html>
		. . .
	</html>

The response tells the client what version of the HTTP specification that it's using and, probably most
importantly, the status code (200). The status code is one of a number of codes that have been standardized
to have a very specific meaning to the client. This can tell them that it was successful (200), or that the page
wasn't found (404). Head over to IANA for a `full list of HTTP status codes
<https://www.iana.org/assignments/http-status-codes/http-status-codes.xhtml>`_.

Working with Requests and Responses
-----------------------------------

While PHP provides ways to interact with the request and response headers, CodeIgniter, like most frameworks,
abstracts them so that you have a consistent, simple interface to them. The :doc:`IncomingRequest class </incoming/incomingrequest>`
is an object-oriented representation of the HTTP request. It provides everything you need::

	use CodeIgniter\HTTP\IncomingRequest;

	$request = new IncomingRequest(new \Config\App(), new \CodeIgniter\HTTP\URI());

	// the URI being requested (i.e. /about)
	$request->uri->getPath();

	// Retrieve $_GET and $_POST variables
	$request->getVar('foo');
	$request->getGet('foo');
	$request->getPost('foo');

	// Retrieve JSON from AJAX calls
	$request->getJSON();

	// Retrieve server variables
	$request->getServer('Host');

	// Retrieve an HTTP Request header, with case-insensitive names
	$request->getHeader('host');
	$request->getHeader('Content-Type');

	$request->getMethod();  // GET, POST, PUT, etc

The request class does a lot of work in the background for you, that you never need to worry about.
The `isAJAX()` and `isSecure()` methods check several different methods to determine the correct answer.

.. note:: The ``isAJAX()`` method depends on the ``X-Requested-With`` header, which in some cases is not sent by default in XHR requests via JavaScript (i.e. fetch). See the :doc:`AJAX Requests </general/ajax>` section on how to avoid this problem.

CodeIgniter also provides a :doc:`Response class </outgoing/response>` that is an object-oriented representation
of the HTTP response. This gives you an easy and powerful way to construct your response to the client::

  use CodeIgniter\HTTP\Response;

  $response = new Response();

  $response->setStatusCode(Response::HTTP_OK);
  $response->setBody($output);
  $response->setHeader('Content-type', 'text/html');
  $response->noCache();

  // Sends the output to the browser
  $response->send();

In addition, the Response class allows you to work the HTTP cache layer for the best performance.
