Programming PHP

The biggest difference between GET and POST requests, however, is far more subtle.
The HTTP specification says that GET requests are idempotent—that is, one
GET request for a particular URL, including form parameters, is the same as two or
more requests for that URL. Thus, web browsers can cache the response pages for
GET requests, because the response page doesn’t change regardless of how many
times the page is loaded. Because of idempotence, GET requests should be used only
for queries such as splitting a word into smaller chunks or multiplying numbers,
where the response page is never going to change.
POST requests are not idempotent. This means that they cannot be cached, and the
server is recontacted every time the page is displayed. You’ve probably seen your
web browser prompt you with “Repost form data?” before displaying or reloading
certain pages. This makes POST requests the appropriate choice for queries whose
response pages may change over time—for example, displaying the contents of a
shopping cart or the current messages in a bulletin board.
That said, idempotence is often ignored in the real world. Browser caches are generally
so poorly implemented, and the Reload button is so easy to hit, that programmers
tend to use GET and POST simply based on whether they want the query
parameters shown in the URL or not. What you need to remember is that GET
requests should not be used for any actions that cause a change in the server, like
placing an order or updating a database.
The type of method that was used to request a PHP page is available through $_
SERVER['REQUEST_METHOD']. For example:
if ($_SERVER['REQUEST_METHOD'] == 'GET') {
// handle a GET request
} else {
die("You may only GET this page.");
}
Parameters
Use the $_POST, $_GET, and $_FILES arrays to access form parameters from your PHP
code. The keys are the parameter names, and the values are the values of those
parameters. Because periods are legal in HTML field names, but not in PHP variable
names, periods in field names are converted to underscores (_) in the array.
Example 7-1 shows an HTML form that chunkifies a string supplied by the user. The
form contains two fields: one for the string (parameter name "word") and one for the
size of chunks to produce (parameter name "number").
Example 7-1. The chunkify form (chunkify.html)
Chunkify Form
This is the Title of the Book, eMatter Edition
Copyright © 2002 O’Reilly & Associates, Inc. All rights reserved.
Processing Forms | 163