The PHP Hypertext Preprocessor (PHP) is a programming language that allows web developers to create dynamic content that interacts with databases. PHP is basically used for developing web based software applications. PHP has a total of eight data types which we use to construct our variables − Integers − are whole numbers, without a decimal point, like 4195. Doubles − are floating-point numbers, like 3.14159 or 49.1. Booleans − have only two possible values either true or false. NULL − is a special type that only has one value: NULL. Strings − are sequences of characters, like 'PHP supports string operations.' Arrays − are named and indexed collections of other values. Objects − are instances of programmer-defined classes, which can package up both other kinds of values and functions that are specific to the class. Resources − are special variables that hold references to resources external to PHP

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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, CSS for presentation, along with Document Object Model and JavaScript for dynamic content display.  Conventional web applications transmit information to and from the sever using synchronous requests. It means you fill out a form, hit submit, and get directed to a new page with new information from the server.  With AJAX, when you hit submit, 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. Clicking is not required, mouse movement is a sufficient event trigger.  Data-driven as opposed to page-driven.

All modern browsers support the XMLHttpRequest object. The XMLHttpRequest object can be used to exchange data with a server behind the scenes. This means that it is possible to update parts of a web page, without reloading the whole page. var xhttp = new XMLHttpRequest(); Send a Request To a Server To send a request to a server, we use the open() and send() methods of the XMLHttpRequest object: xhttp.open("GET", "ajax_info.txt", true); xhttp.send();

onreadystatechange Defines a function to be called when the readyState property changes readyState Holds the status of the XMLHttpRequest.  If readyState= 0: request not initialized  1: server connection established 2: request received  3: processing request  4: request finished and response is ready