Operators in PHP

The most common PHP operators are 

1. Assignment operators,
2. Arithmetic operators, 
3. combined operators, 
4. comparison operators, 
5. logical operators. 
6. Relational Operators
7. Increment and Decrement Operator
8. Negation Operator
9. Error Controlling operator
10. String/Concatenation operator
11. Conditional(or) Ternary Operator
12. "new" Operator
13. "instanceOf" operator
14. Scope resolution operator
15. Dereferencing operator

Assignment Operators

The basic assignment operator in PHP is "=". This means that the operand to the left of "=" gets set to the value to the right of "=".

Arithmetic Operators

Operator	Example	Result
+	4 + 2	6
-	4 - 2	2
*	4 * 2	8
/	4 / 2	2
%	4 % 2	0
++	x = 4; x++;	x = 5
--	x = 4; x--;	x = 3

Combined Operators

You can combine an arithmetic operator with the assignment operator to form a combined operator. Combined operators are shown below:

Operator	Example	Meaning
+=	y += x	y = y + x
-=	y -= x	y = y - x
*=	y *= x	y = y * x
/=	y /= x	y = y / x
%=	y %= x	y = y % x

Comparison Operators

Operator	Meaning
==	is equal to
!=	is not equal to
>	is greater than
>=	is greater than or equal to
<	is less than
<=	is less than or equal to

Logical Operators

Operator	Meaning
||	or
&&	and
and	and
or	or
xor	xor
!	not

Relational Operators
= = = will check for the content and also for the type.
! = =  


Increment and Decrement Operator


++ and - -


Negation Operator


- multiplying with (-1)


Error Controlling Operator:


@   when prepended an expression, any warning messages that is being generated will gets suppressed.


A PHP application is associated with two types of runtime mistakes:

• Warning - if  warning occurs, the rest of the Programme  will be executed.
• A Fatal error - if a fatal error occurs the Programme gets terminated.

String/concatenation operators:

. or .=

Conditional / ternery operator:   ?:

Variables in PHP

Variable Types

Variables play an important role in PHP, as they are places for holding values. In PHP, there is no need to declare variables. Variable can hold eight different data types:

•  bloolean,
•  integer, 
• float, 
• string, 
• array,
•  object,
•  resource, 
• NULL.

PHP is a weakly typed language. This means that variable type varies depending on what is stored in the variable at the time. For example, if we have a variable $a, when $a = 0, $a is an integer type variable. If later we set $a = "New", then $a becomes a string type variable.

Variable Name:

A variable name always starts with a $, followed by a letter or an underscore. The rest of the variable name can be a letter, a number or an underscore.

For example, $dog is a valid variable name, while @dog is not (@dog does not start with a $).

Variables in PHP are case-sensitive. For example, $Employee and $employee are two different variables.

Variable Scope:

In most cases, variables are local in scope. This means that variables declared within a function cannot be accessed outside of the function, and variables declared outside of a function cannot be access within the function.

To make a variable global, it must either be declared as global specifically, or it must be accessed using the $GLOBALS array.

XMLHttpRequest Properties

• onreadystatechange
  An event handler for an event that fires at every state change.
• readyState
  The readyState property defines the current state of the XMLHttpRequest object.
  Here are the possible values for the readyState propery:

  State	Description
  0	The request is not initialized
  1	The request has been set up
  2	The request has been sent
  3	The request is in process
  4	The request is completed

  readyState=0 after you have created the XMLHttpRequest object, but before you have called the open() method.
  readyState=1 after you have called the open() method, but before you have called send().
  readyState=2 after you have called send().
  readyState=3 after the browser has established a communication with the server, but before the server has completed the response.
  readyState=4 after the request has been completed, and the response data have been completely received from the server.
• responseText
  Returns the response as a string.
• responseXML
  Returns the response as XML. This property returns an XML document object, which can be examined and parsed using W3C DOM node tree methods and properties.
• status
  Returns the status as a number (e.g. 404 for "Not Found" and 200 for "OK").
• statusText
  Returns the status as a string (e.g. "Not Found" or "OK").

XMLHttpRequest Methods

abort(): Cancels the current request.
getAllResponseHeaders(): Returns the complete set of HTTP headers as a string.
getResponseHeader( headerName ): Returns the value of the specified HTTP header.
open( method, URL )
open( method, URL, async )
open( method, URL, async, userName )
open( method, URL, async, userName, password ): Specifies the method, URL, and other optional attributes of a request.

The method parameter can have a value of "GET", "POST", or "HEAD". Other HTTP methods, such as "PUT" and "DELETE" (primarily used in REST applications), may be possible

The "async" parameter specifies whether the request should be handled asynchronously or not . "true" means that script processing carries on after the send() method, without waiting for a response, and "false" means that the script waits for a response before continuing script processing.

send( content ): Sends the request.
setRequestHeader( label, value ) :Adds a label/value pair to the HTTP header to be sent.

XMLHTTPRequest

The XMLHttpRequest object is the key to AJAX. It has been available ever since Internet Explorer 5.5 was released in July 2000, but not fully discovered before people started to talk about AJAX and Web 2.0 in 2005.

XMLHttpRequest (XHR) is an API that can be used by JavaScript, JScript, VBScript and other web browser scripting languages to transfer and manipulate XML data to and from a web server using HTTP, establishing an independent connection channel between a web page's Client-Side and Server-Side.

The data returned from XMLHttpRequest calls will often be provided by back-end databases. Besides XML, XMLHttpRequest can be used to fetch data in other formats, e.g. JSON or even plain text.

Steps to Do Ajax Programming

1.A client event occurs

• A JavaScript function is called as the result of an event
• Example: validateUserId() JavaScript function is mapped as a event handler to a onkeyupevent on input form field whose id is set to "userid"

2. The XMLHttpRequest object is created
      
var ajaxRequest;   // The variable that makes Ajax possible!
function ajaxFunction(){
try{
               // Opera 8.0+, Firefox, Safari
ajaxRequest = new XMLHttpRequest();
}catch (e){
            // Internet Explorer Browsers
try{
ajaxRequest = new ActiveXObject("Msxml2.XMLHTTP");
}catch (e) {
try{
ajaxRequest = new ActiveXObject("Microsoft.XMLHTTP");
}catch (e){
                // Something went wrong
alert("Your browser broke!");
return false;
}
}
}
}
 
3. The XMLHttpRequest object is Configured
In this step we will write a function which will be triggered by the client event and a callback function processRequest() will be registered

function validateUserId() {
ajaxFunction();
  // Here processRequest() is the callback function.
ajaxRequest.onreadystatechange = processRequest;
if (!target) target = document.getElementById("userid");
var url = "validate?id=" + escape(target.value);
ajaxRequest.open("GET", url, true);
ajaxRequest.send(null);
}

4. Making Asynchornous Request to the Webserver
Source code is available in the above piece of code. Code written in blue color is responsible to make a request to the web server. This is all being done using XMLHttpRequest objectajaxRequest

function validateUserId() {
ajaxFunction();
         // Here processRequest() is the callback function.
ajaxRequest.onreadystatechange = processRequest;
if (!target) target = document.getElementById("userid");
var url = "validate?id=" + escape(target.value);
ajaxRequest.open("GET", url, true);
ajaxRequest.send(null);
}

Assume if you enter srinivas in userid box then in the above request URL is set to validate?id=srinivas

5. Webserver returns the result containing XML document
You can implement your server side script in any language. But logic should be as follows

• Get a request from the client
• Parse the input from the client
• Do required processing.
• Send the output to the client.

If we assume that you are going to write a servlet then here is the piece of code

public void doGet(HttpServletRequest request, HttpServletResponse response)throws IOException, ServletException
{
String targetId = request.getParameter("id");
if ((targetId != null) &&
!accounts.containsKey(targetId.trim()))
{
response.setContentType("text/xml");
response.setHeader("Cache-Control", "no-cache");
response.getWriter().write("true");
}
else
{
response.setContentType("text/xml");
response.setHeader("Cache-Control", "no-cache");
response.getWriter().write("false");
}
}
 
 6. Callback function processRequest() is called
The XMLHttpRequest object was configured to call the processRequest() function when there is a state change to the readyState of the XMLHttpRequest object. Now this function will recieve the result from the server and will do required processing. As in the following example it sets a variable message on true or false based on retruned value from the Webserver.
 
function processRequest() {
if (req.readyState == 4) {
if (req.status == 200) {
var message = ...;
}

7. The HTML DOM is updated
This is the final step and in this step your HTML page will be updated. It happens in the following way JavaScript technology gets a reference to any element in a page using DOM API
The recommended way to gain a reference to an element is to call.

document.getElementById("userIdMessage"),
     // where "userIdMessage" is the ID attribute
    // of an element appearing in the HTML document
JavaScript technology may now be used to modify the element's attributes; modify the element's style properties; or add, remove, or modify child elements. Here is the example

script type="text/javascrip
function setMessageUsingDOM(message) {
var userMessageElement = document.getElementById("userIdMessage");
var messageText;
if (message == "false") {
userMessageElement.style.color = "red";
messageText = "Invalid User Id";
} else {
userMessageElement.style.color = "green";
messageText = "Valid User Id";
}
var messageBody = document.createTextNode(messageText);
       // if the messageBody element has been created simple
      // replace it otherwise append the new element
if (userMessageElement.childNodes[0]) {
userMessageElement.replaceChild(messageBody, userMessageElement.childNodes[0]);
} else {
userMessageElement.appendChild(messageBody);
}
}
start body
div id="userIdMessage"
end of body

How to Define Ajax Programme

Steps of AJAX Operation

1. A client event occurs
2. An XMLHttpRequest object is created
3. The XMLHttpRequest object is configured
4. The XMLHttpRequest object makes an asynchronous request to the Webserver.
5. Webserver returns the result containing XML document.
6. The XMLHttpRequest object calls the callback() function and processes the result.
7. The HTML DOM is updated

Browser support for Ajax

All the available browsers can not support AJAX. Here is the list of major browsers which support AJAX.

• Mozilla Firefox 1.0 and above
• Netscape version 7.1 and above
• Apple Safari 1.2 and above.
• Microsoft Internet Exporer 5 and above
• Konqueror
• Opera 7.6 and above

So now when you write your application then you would have to take care of the browsers who do not support AJAX.

NOTE: When we are saying that browser does not support AJAX it simply means that browser does not support creation of Javascript object XMLHttpRequest object.
 
Writing Browser Specific Code
 Simple way of making your source code compatible to a browser is to use try...catch blocks in your javascript.

Name: Time:  

In the above Javascript code, we try three times to make our XMLHttpRequest object. Our first attempt:

ajaxRequest = new XMLHttpRequest();

is for the Opera 8.0+, Firefox and Safari browsers. If that fails we try two more times to make the correct object for an Internet Explorer browser with:
 
ajaxRequest = new ActiveXObject("Msxml2.XMLHTTP");
ajaxRequest = new ActiveXObject("Microsoft.XMLHTTP");>
If that doesn't work, then they are using a very outdated browser that doesn't supportXMLHttpRequest, which also means it doesn't support Ajax.

Most likely though, our variable ajaxRequest will now be set to whatever XMLHttpRequeststandard the browser uses and we can start sending data to the server.

Technologies used in Ajax

JavaScript

• Loosely typed scripting language
• JavaScript function is called when an event in a page occurs
• Glue for the whole AJAX operation

DOM

• API for accessing and manipulating structured documents
• Represents the structure of XML and HTML documents

CSS

• Allows for a clear separation of the presentation style from the content and may be changed pro-grammatically by JavaScript

 XMLHttpRequest

• JavaScript object that performs asynchrous interaction with the server

Ajax Introduction

• AJAX stands for Asynchronous JavaScript and XML. AJAX is a new technique for creating better, faster, and more interactive web applications with the help of XML, HTML, CSS and Java Script.
• Ajax uses XHTML for content and CSS for presentation, as well as the Document Object Model and JavaScript for dynamic content display.
• Conventional web application trasmit information to and from the sever using synchronous requests. This means you fill out a form, hit submit, and get directed to a new page with new information from the server.
• With AJAX when submit is pressed, JavaScript will make a request to the server, interpret the results and update the current screen. In the purest sense, the user would never know that anything was even transmitted to the server.
• XML is commonly used as the format for receiving server data, although any format, including plain text, can be used.
• AJAX is a web browser technology independent of web server software.
• A user can continue to use the application while the client program requests information from the server in the background
• Intuitive and natural user interaction. No clicking required only Mouse movement is a sufficient event trigger.
• Data-driven as opposed to page-driven

AJAX is based on the following open standards:

• Browser-based presentation using HTML and Cascading Style Sheets (CSS)
• Data stored in XML format and fetched from the server
• Behind-the-scenes data fetches using XMLHttpRequest objects in the browser
• JavaScript to make everything happen

How do I use a scriptlet to initialize a newly instantiated bean?

jsp:useBean action may optionally have a body. If the body is specified, its contents will be automatically invoked when the specified bean is instantiated. Typically, the body will contain scriptlets or jsp:setProperty tags to initialize the newly instantiated bean, although you are not restricted to using those alone.

The following example shows the “today” property of the Foo bean initialized to the current date when it is instantiated. Note that here, we make use of a JSP expression within the jsp:setProperty action.