Page 527 - Beginning PHP 5.3

Chapter 16: PHP and the Outside World
In the following sections you explore how a Web browser makes an HTTP request to the Web server;
how the server then sends an HTTP response back to the browser; and how you can influence the HTTP
communication between server and browser.

Understanding HTTP Requests

Whenever a browser wants to display a Web page or other resource (such as an image file) that is stored
on a Web server, the browser first connects to the server (usually via port 80, the HTTP port), and then
sends various pieces of information to the server, known as the request message . The request message
consists of the following sections, in order:

❑ The request line: This tells the Web server which resource (URL) the browser wants to retrieve
❑ A list of HTTP headers: These optional lines of text allow the browser to send additional
information to the server, such as cookies and which character sets the browser can accept
❑ An empty line: This is required after the request line and any headers
❑ An optional message body: This might contain, for example, form data sent via the POST
method

Each line in the message must end with a carriage return character followed by a line feed character.
The request line is the most important part of the request, because it tells the server which resource to
send back to the browser. Here ’ s a typical request line:

GET /about/index.php HTTP/1.1
The request line consists of three parts: The request method ( GET in this case), the URL to retrieve (/
about/index.php ), and the version of HTTP to use (most modern browsers work with HTTP/1.1 ).
Other request methods include POST (for sending large amounts of form data) and HEAD (similar to GET
but asks the server to return just the response headers, rather than the actual content).

Many HTTP request headers can be sent from a browser to a server. Here are some common ones:

Header Description Example

Accept A list of MIME content types that the Accept: text/html,
browser will accept for the returned application/xml
content.

Accept - Charset A list of character sets that the browser Accept - Charset:
will accept for the returned content. ISO - 8859 - 1,utf -
8
Accept - Encoding A list of compression methods that the Accept - Encoding:
browser will accept for the returned gzip,deflate
content.

Accept - Language A list of languages that the browser will Accept - Language:
accept for the returned content. en - gb,en

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