what is AJAX?

AJAX is Asynchronous JavaScript and XML.

A scripting technique for silently loading new data / web page from the server. Although AJAX scripts commonly use the soon to be standardized XMLHttpRequest object, they could also use a hidden iframe or frame. An AJAX script is useless by itself. It also requires a DOM Scripting component to embed the received data in the document.

In another way we can say Ajax, shorthand for Asynchronous JavaScript and XML, is a Web development technique for creating interactive web applications. The intent is to make web pages feel more responsive by exchanging small amounts of data with the server behind the scenes, so that the entire web page does not have to be reloaded each time the user makes a change. This is meant to increase the web page's interactivity, speed, and usability. The first known use of the term in public was by Jesse James Garrett in his February 2005 article Ajax: A New Approach to Web Applications. At subsequent talks and seminars Garrett has made the point that Ajax is not an acronym.



History of AJAX

Although the term 'Ajax' was coined in 2005, most histories of the technologies that enable Ajax start a decade earlier with Microsoft's initiatives in developing Remote Scripting. However techniques for the asynchronous loading of content on an existing web page without requiring a full reload date back as far as the IFRAME element type (introduced in Internet Explorer 3 in 1996) and the LAYER element type (introduced in Netscape 4 in 1997, abandoned during early development of Mozilla). Both element types had a src attribute that could take any external URL, and by loading a page containing JavaScript that manipulated the parent page, Ajax-like effects could be attained.

Microsoft's Remote Scripting (or MSRS, introduced in 1998) acted as a more elegant replacement for these techniques, with data being pulled in by a Java applet with which the client side could communicate using JavaScript. This technique worked on both Internet Explorer version 4 and Netscape Navigator version 4 onwards. Microsoft took first advantage of these techniques in Outlook Web Access supplied with the Microsoft Exchange Server 2000 release.

The web development community, first collaborating via the microsoft.public.scripting.remote newsgroup and later through blog aggregation, subsequently developed a range of techniques for remote scripting in order to enable consistent results across different browsers. In 2002, a user-community modification to Microsoft Remote Scripting was made to replace the Java applet with XMLHttpRequest.

Remote Scripting Frameworks such as ARSCIF surfaced in 2003 not long before Microsoft introduced Callbacks in ASP.NET.

Since XMLHttpRequest is now implemented across the majority of browsers in use, alternative techniques are used infrequently. However, they are still used where wide compatibility, small implementation, or cross-site access are required.


Pros, cons, and criticism

Pros

Interactivity
Ajax applications are mainly executed on the user's machine, by manipulating the current page within their browser using document object model methods. Ajax can be used for a multitude of tasks such as updating or deleting records; expanding web forms; returning simple search queries; or editing category trees—all without the requirement to fetch a full page of HTML each time a change is made. Generally only small requests are required to be sent to the server, and relatively short responses are sent back. This permits the development of more interactive applications featuring more responsive user interfaces due to the use of DHTML techniques.

While the Ajax platform is more restricted than the Java platform, current Ajax applications effectively fill part of the one-time niche of Java applets: extending the browser with lightweight mini-applications.


Cons and criticism

Usability criticisms
One major complaint voiced against the use of Ajax in web applications is that it might easily break the expected behavior of the browser's back button. The different expectations between returning to a page which has been modified dynamically versus the return to a previous static page might be a subtle one. Users generally expect that clicking the back button in web applications will move to their last page loaded, and in Ajax applications this might not be the case. Developers have implemented various solutions to this problem, most of which revolve around creating or using invisible IFRAMEs to invoke changes that populate the history used by a browser's back button. Google Maps, for example, performs searches in an invisible IFRAME and then pulls results back into an element on the visible web page; it is possible to track user behaviour via callbacks which are called whenever the back button is pressed, restoring the application state that existed at the time.

A related issue is that dynamic web page updates make it difficult for a user to bookmark a particular state of the application. Solutions to this problem exist, many of which use the URL fragment identifier (the portion of a URL after the '#' [1] [2]) to keep track of, and allow users to return to, the application in a given state. This is possible because many browsers allow JavaScript to update the fragment identifier of the URL dynamically, so that Ajax applications can maintain it as the user changes the application's state. This solution also improves back-button support.


Response-time concerns
Network latency — or the interval between user request and server response — needs to be considered carefully during Ajax development. Without clear feedback to the user [3], smart preloading of data and proper handling of the XMLHttpRequest object, users might experience delay in the interface of the web application, something which users might not expect or understand [4]. The use of visual feedback to alert the user of background activity and/or preloading of content and data are often suggested solutions to these latency issues.

In general the potential impact of latency has not been "solved" by any of the open source Ajax toolkits and frameworks available today, such as the effect of latency variance over time.


JavaScript
While no browser plug-in is required for Ajax, it requires users to have JavaScript enabled in their browsers. This applies to all browsers that support Ajax except for Microsoft Internet Explorer 6 and below which additionally require ActiveX to be enabled, as the XMLHTTP object is implemented with ActiveX in this browser. Internet Explorer 7, however, will implement this interface as a native JavaScript object and hence does not need ActiveX to be enabled for Ajax to work.

As with DHTML applications, Ajax applications must be tested rigorously to deal with the quirks of different browsers and platforms. A number of programming libraries have become available as Ajax has matured that can help ease this task. Likewise, techniques have been developed to assist in designing applications which degrade gracefully and offer alternative functionality for users without JavaScript enabled.


Name issues
There have been some critics of the term Ajax, claiming that Adaptive Path (the consulting firm that coined the term [5]) or other proponents are using it as a marketing vehicle for previously-used techniques.


Accessibility
Using Ajax technologies in web applications provides many challenges for developers interested in adhering to WAI accessibility guidelines. In addition there are numerous development groups working on government projects which require strict adherence to Section 508 Compliance standards. Failure to comply with these standards can often lead to cancellation of contracts or lawsuits intended to ensure compliance.

Because of this, developers need to provide fallback options for users on other platforms or browsers, as most methods of Ajax implementation rely on features only present in desktop graphical browsers.

Web developers use Ajax in some instances to provide content only to specific portions of a web page, allowing data manipulation without incurring the cost of re-rendering the entire page in the web browser. Non-Ajax users would ideally continue to load and manipulate the whole page as a fallback, allowing the developers to preserve the experience of users in non-Ajax environments (including all relevant accessibility concerns) while giving those with capable browsers a much more responsive experience.



That all AJAX is. in this article some of the content is importent for new programmer

PHP installation on Windows

Installation on Windows systems
This section applies to Windows 98/Me and Windows NT/2000/XP. PHP will not work on 16 bit platforms such as Windows 3.1 and sometimes we refer to the supported Windows platforms as Win32. Windows 95 is no longer supported as of PHP 4.3.0.

There are two main ways to install PHP for Windows: either manually or by using the InstallShield installer.

If you have Microsoft Visual Studio, you can also build PHP from the original source code.

Once you have PHP installed on your Windows system, you may also want to load various extensions for added functionality.

Windows InstallShield
The Windows PHP installer is available from the downloads page at http://www.php.net/downloads.php. This installs the CGI version of PHP and, for IIS, PWS, and Xitami, configures the web server as well.

Note: While the InstallShield installer is an easy way to make PHP work, it is restricted in many aspects, as automatic setup of extensions for example is not supported. The whole set of supported extensions is only available by downloading the zip binary distribution.

Install your selected HTTP server on your system and make sure that it works.

Run the executable installer and follow the instructions provided by the installation wizard. Two types of installation are supported - standard, which provides sensible defaults for all the settings it can, and advanced, which asks questions as it goes along.

The installation wizard gathers enough information to set up the php.ini file and configure the web server to use PHP. For IIS and also PWS on NT Workstation, a list of all the nodes on the server with script map settings is displayed, and you can choose those nodes to which you wish to add the PHP script mappings.

Once the installation has completed the installer will inform you if you need to restart your system, restart the server, or just start using PHP.