Understanding HTTP Requests: URL Variables, Form Data, and Server Processing, Slides of Web Design and Development

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HTML Form Processing

Web Client-Server Model

http response

http request

Specify the URL of the “file” you want.

Send back the “file” to the client.

What is in an http request?

• Information about the client browser

• Firefox, IE, Safari, etc.
• Browser Version
• Platform (Windows, Unix, MacOS, etc.)

• Also the URL itself can include variables

• http://www.site.com/makepage.php ?pageid=48&display=wide

URL Variables

http://www.site.com/makepage.php ?pageid=48&display=wide

•? Defines the URLs query string

• pageid and display are query variables

• & is a delimiter, i.e., put between query

variables

HTML Forms

• Consider the following HTML code

Enter Email:

• When you click the submit button, the browser

• requests the file “ script.php ”
• and sends the server the data that user enters

email = “[email protected]”

Summary: Data sent by client

1. URL itself

2. URL variables

3. Information about the client browser

4. Any form data

Variables provided by the server.

• $_GET – stores all the URL variables

• $_POST – stores all the Form variables

• $_SERVER – stores server information

including info about the client browsers that

just made the request.

Example

• http://www.site.com/add.php? xval=5 & yval=

• add.php

<?php

$xValue = $_GET['xval'];

$yValue = $_GET['yval'];

$sum = $xValue + $yValue;

echo $xValue,' + ', $yValue, ' = ', $sum;

Get vs. Post

$sum = $_POST ['xval'] + $_POST ['yval']

echo $_POST ['xval'],' + ', $_POST ['yval'], ' = ', $sum;

?>

+

4 5

GET vs. POST

GET – First way

• Browser adds the form variables to the URL
• Then sends the http request
• Form variables are visible in the URL
• Browsers limit the length a URL
• Limit for IE is 2, characters

POST – Improved way

• Browser adds the form variables to the message body of the http request
• Then sends the http request
• Must be used when sending non-text data, binary files, images, etc.
• Must be used when sending long data (> 2KB)

One more look

$sum = $_POST ['xval'] + $_POST ['yval']

echo $_POST ['xval'],' + ', $_POST ['yval'], ' = ', $sum;

?>

+

4 5

Quirks of Web-page Based Interfaces

• Ever notice the delay in submitting a form?
• Ever notice that forms tend to always send

back a confirmation message, even when you don’t need one?

• Ever notice the page entirely refreshes

whenever a button is clicked?

Synchronous Communication

• A Phone conversation is synchronous
  • Generally, people wait for an answer before asking the next question
  • You talk, I talk, you talk, I talk, …
• Web analogy
  • When submitting a form, you have to wait for a response before you can move to the next action.
• This is why web-interfaces are sometimes quirky:
  • We are used to asynchronous interaction.
  • Web client-server interaction is actually synchronous.
  • Note many “experts” mistakenly consider web client- server interaction to be asynchronous. They are wrong!

Asynchronous Communication

• Email is a classic example of asynchronous

communication

• You do NOT have to wait for an email reply to send

out other emails.

• Web client-server analogy
  • You do NOT have to wait for an http response to send

out another request, even to the same server

• When you look at it this way, web client-server

interaction seems asynchronous, but it is not.