XMLHttpRequest and AJAX for PHP programmers ...

Introduction:
Although the concept isn entirely new, XMLHttpRequest technology is implimented on more sites now than ever. Compatibility is no longer an issue (IE, Mozilla and Opera all support it), and the benefits to using it are amazing. There are too many PHP programmers avoiding any work with JavaScript beyond simple form validation, and for good reason. It is difficult to keep several languages proficiently under your belt. But using the XMLHttpRequest object is not as hard as everybody thinks, and you don need to buy and memorize another reference manual.
Let is Get To It!
Asynchronous JavaScript and XML, or AJAX is a method of sending and receiving data (usually XML) from a server-side application through javascript. Since javascript offers the ability to change the contents of a web page on-the-fly, this technique allows web programmers to venture closer to programming truly interactive web applications similar to those built with Java and ActiveX.
As PHP developers, it might seem tempting to avoid the use of Javascript and leave it to the dsigner. After all, we aren usually programming the UI, but the processing components required by the UI. The distinction between the two is disappearing. Here is a simple diagram that demonstrates just how AJAX works:

If you are working for a small or medium sized company interested in implementing AJAX solutions, you might end up responsible for figuring out how.
XMLHttpRequest objects can be a simple way of getting data to and from a PHP application while keeping your client right at home on the same page. Our example today will allow a user to select a specific piece of software that your company makes. We will show a selection box with several categories. When a user selects a category, a request is sent to a PHP application which returns a list of applicable software. The information is used to generate a list of the results underneath the selection box. Since the information is not loaded with the initial page, your company saves bandwidth, and because the user doesn have to bounce from page to page for results, he will find your company is page more inviting and faster to load.
Javascript for PHP Programmers
Since we will be working with Javascript, it is good to get a basic tutorial given a background in PHP.
Variables:
Variables in javascript are declared in much the same was as in PHP. To declare a variable in javascript, use the following:

[javascript code]
var varname = varvalue; //Declaring your variable is not mandatory, but good practice.

Variables:
Variable types are handled loosely, as they are in PHP.
Control Structures:
PHP and javascript have a very similar way of using these as well... You can use if/else statements, switch statements, for and while loops and nested loops all with PHP syntax. If/elseif/else statements are a little different, but not much:

[javascript code]
var variable1 = 1; //Declare a variable.
if(variable1 == 1){
 // Increment variable value by 1
 variable1++;
 /* The following brings up a message box, a handy way of checking variable values as you
        go. Be careful not to use these in loops that are too long. */
 alert(variable1);
}else if(variable1 == 2){ //Elseif is in javascript require a space between the else and if.
 for(i=0;i<20;i++){ //For loop.
  variable1++;
 }
     switch(variable1){ //Switch conditional
         case 22:
             alert("variable 1 has value of 22!");
             break;
         default:
             alert("didn have value of 22!");
   break;
 }
}else{
 alert(The final else statement);
}

Functions
To declare a function in javascript, use the following:

[javascript code]
function functioname(argument1, argument2, argument3){
 /* Arguments not provided will be null */
 if(argument3 == null){
  return argument1 + argument2
 }else{
  return argument1 + argument2 + argument3
 }
}

/* And call it with */
alert(functionname(1, 2)); //returns 3
/* or */
alert(functionname(1, 2, 3)); //returns 6

Other than the difference with default values for optional arguments, you should assign a default when a passed argument has a null value.
Using Javascript in a Document:
Javascript can be included in your document in two ways, similar to CSS inclusion:

[HTML code]
<!--Placing your javascript in the head tag of your html document is the standard-->
<script language="javascript" type="text/javascript" src="./javascript_file.js"></script>
<script language="javascript" type="text/javascript">

 /*Javscript code goes in here*/
 alert("Hello World!");
</script>

The first way is to link to the file directly, and the second is to include it on the page itself. If your code is getting long and is used on multiple pages, it is preferable to use the first method, as the browser will cache the file for future use. And that is all you need to know about Javascript to continue.
Developing the Initial Page
Below we are going to be creating the page the user loads to view your companies products.

[HTML code]
<html>
 <head>
  <title>CompanyXYZ Software</title>
  <script language="javascript" type="text/javascript"
   src="./internal_request.js">

  </script>
 </head>
 <body>
  <div id="product_categories">
   <!--Category selection box...-->
   <form name="form_category_select">

    <select>
     <option>Audio</option>
     <option>Games</option>
     <option>Internet</option>

    </select>
   </form>
  </div>
  <div id="product_cage">
   <!--This is where we will be displaying the products once they are loaded-->
   ^ Please select a category from above.
  </div>

 </body>
</html>

The above HTML first links to a javascript file, internal_request.js, and displays a page with two <div> tags, the second of which is where we will be displaying our data. Go ahead and copy the above code into a file called products.html.
Creating the XMLHttpRequest Object
The XMLHttpRequest object works differently in Internet Explorer and Mozilla-like browsers. To create an XMLHttpRequest object in IE, the following can be used:

[javascript code]
var request_o = new ActiveXObject("Microsoft.XMLHTTP");

And the following works for supporting browsers other than IE:

[javascript code]
var request_o = new XMLHttpRequest();

Determining what browser you are working with and creating the appropriate object is simple:

[javascript code]
/* The following function creates an XMLHttpRequest object... */

function createRequestObject(){
 var request_o; //declare the variable to hold the object.
 var browser = navigator.appName; //find the browser name
 if(browser == "Microsoft Internet Explorer"){
  /* Create the object using MSIE is method */
  request_o = new ActiveXObject("Microsoft.XMLHTTP");
 }else{
  /* Create the object using other browser is method */
  request_o = new XMLHttpRequest();
 }
 return request_o; //return the object
}

/* You can get more specific with version information by using
 parseInt(navigator.appVersion)
 Which will extract an integer value containing the version
 of the browser being used.
*/
 

Copy the above code into a file called internal_request.js, located in the same directory as the products.html file.

We now have a function that will create an XMLHttpRequest object in internal_request.js, and we have an HTML file that calls upon the code in internal_request.js. Remember how we left the product selection <div> in products.html blank? Let is write the code that utilizes our createRequestObject function to get the list of products.

[javascript code]

/* The variable http will hold our new XMLHttpRequest object. */
var http = createRequestObject();

/* Function called to get the product categories list */
function getProducts(){
 /* Create the request. The first argument to the open function is the method (POST/GET),
  and the second argument is the url...
  document contains references to all items on the page
  We can reference document.form_category_select.select_category_select and we will
  be referencing the dropdown list. The selectedIndex property will give us the
  index of the selected item.
 */
 http.open(get, internal_request.php?action=get_products&id=
   + document.form_category_select.select_category_select.selectedIndex);
 /* Define a function to call once a response has been received. This will be our
  handleProductCategories function that we define below. */
 http.onreadystatechange = handleProducts;
 /* Send the data. We use something other than null when we are sending using the POST
  method. */
 http.send(null);
}

/* Function called to handle the list that was returned from the internal_request.php file.. */
function handleProducts(){
 /* Make sure that the transaction has finished. The XMLHttpRequest object
  has a property called readyState with several states:
  0: Uninitialized
  1: Loading
  2: Loaded
  3: Interactive
  4: Finished */
 if(http.readyState == 4){ //Finished loading the response
  /* We have got the response from the server-side script,
   let is see just what it was. using the responseText property of
   the XMLHttpRequest object. */
  var response = http.responseText;
  /* And now we want to change the product_categories <div> content.
   we do this using an ability to get/change the content of a page element
   that we can find: innerHTML. */
  document.getElementById(product_cage).innerHTML = response;
 }
}

The above code should be appended to what you already have in the internal_request.js file.

 Original Source:
http://www.phpbuilder.com/columns/kassemi20050606.php3