In this post, we talk about Ajax (Asynchronous JavaScript and XML) in the LAMP stack.
 Ajax helps make calls to the server to update a page without a page refresh. This allows client side of the application to remain responsive even when the data is exchanged with the server.
 With page refreshes, the entire page was being transmitted several times consuming bandwidth, transmitting same elements over and over again and making the web application unresponsive during transfers. With Ajax, the page is responsive to user actions during transfers.
Ajax allows rich fat clients to be written. By that we mean the client is capable of a lot of functionality. Take Gmail for instance. It's a web client that uses Ajax.  The page isn't refreshed but the elements are manipulated via JavaScript with the application and XML communication with the GMail Server.
Prior to Ajax, the techniques used to communicate with the server without refresh included the following: The image source trick - with the src property of an image in the HTML, developers manipulated the brower's behavior to send a web request for the image and to package its response with a server side script to access a data store. However it was one-way and it wasn't robust because it couldn't handle errors.
Hidden Frames was another technique where the developers used it to send and receive information from the web server It's simple to use and debug and the frame load adds a browser history entry.
The downsides are that the user can manipulate the content.
Hidden iFrames instead of Frames help alleviate the issues with the behavior of the back button.
XMLHttp and XMLHttpRequest interfaces enables the following methods that are supported in all browsers.
abort() - aborts a request,
getResponseHeader(key) : returns the value of the HTTP response header named key
open(method, uri) : sets the HTTP method and the request URI
overrideMimeType(type): Forces the interpretation of the response
send(requestcontent) sends the HTTP request
setRequestHeader(key, value) assigns header elements and called before open or send
Since the interface goes by different names and is instantiated differently for different browsers, the AJAX code handles this by using the native XMLHttpRequest or the window.ActiveXObject for instantiating the interfaces.
Similarly the status code from the request is checked before the response is parsed.
The responseXML property of the request is parsed and elements may be added to the DOM. Knowing the XML structure may or may not be required. For example, handlers can be written at the level of an XML element instead of the response level.
Some common AJAX libraries are : SAJAX (Simple AJAX toolkit) which allows functions to be defined in PHP and called from the client. CPAINT which is the cross-platform Asynchronous interface toolkit that supports both plain text and XML response type and JPSPAN the bridges the gap between JavaScript and PHP.  It allows for a seamless importing of PHP classes to JavaScript.