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Posted by geevaa on 03/27/07 11:47
http://www.phpbuilder.com/columns/kassemi20050606.php3
XMLHttpRequest and AJAX for PHP programmers
James Kassemi
Introduction:
Although the concept isn't entirely new, XMLHttpRequest technology is
implemented 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's difficult to keep several languages proficiently under
your belt. But using the XMLHttpRequest object is not as hard as
everybody thinks, and you don't need to buy and memorize another
reference manual.
Let's 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't usually
programming the UI, but the processing components required by the UI.
The distinction between the two is disappearing. Here's a simple
diagram that demonstrates just how AJAX works:
If you're 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't have
to bounce from page to page for results, he will find your company's
page more inviting and faster to load.
Javascript for PHP Programmers
Since we'll be working with Javascript, it's 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's 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't 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's all you
need to know about Javascript to continue.
Developing the Initial Page
Below we're 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'll be displaying the
products once they're
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'll 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's method */
request_o = new ActiveXObject("Microsoft.XMLHTTP");
}else{
/* Create the object using other browser's 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's 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's 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.
Conclusion
We'll followup with the rest of this informative article next week, so
be sure to visit us again for the conclusion! btw, don't miss the
important Quick Tips listed below!
Quick Tip 1: (Using the POST method instead of GET):
The following will send the request to the PHP file using the POST
method:
http.abort;
http.open('post', 'back_end.php');
http.setRequestHeader('Content-Type', 'application/x-www-form-
urlencoded');
http.send('arg1=val1&arg2=val2&arg3=val3');
Quick Tip 2: (Parsing XML results):
If you're receiving XML instead plain-text results from your
processing PHP script, you can use the javascript DOM to parse them.
Suppose you receive the following:
<product_list>
<product id="1">
<name>EeasySMS</name>
<version>2.2</version>
</product>
<product id="2">
<name>BabyMon</name>
<version>1.2</version>
</product>
</product_list>
To receive a DOM compatible response from our XMLHttpRequest object,
instead of using the responseText property, substitute the responseXML
property. In this case we'll refer to it by assigning it the variable
XMLResponse.
The product_list element contains two elements of interest: product
elements. In order to get to these, we can use the following:
/* Reference the product_list element to the variable product_list */
product_list = XMLReponse.getElementByTagName('product_id');
/* By substituting product_list for XMLResponse, we will be searching
only the product_list element, not the entire response
We also use getElementsByTagName, not getElementByTagName,
since we are interested in all of the results, not just one.
*/
product_id = XMLResponse.getElementsByTagName('product');
/* getElementsByTagName produces an array, which we can access like
this:
product_id[n], the same way we access an array item in PHP.
Let's get the id attribute from the product elements like
this: */
for(i=0; i<product_id.length; i++){ //length is the same as
count($array)
id = product_id[i].getAttribute('id') //Grabs the id
attribute.
/* To get the text from within a text node, we use
firstChild.data
for the corresponding element. */
name =
product_id[i].getElementByTagName('name').firstChild.data;
version =
product_id[i].getElementByTagName('version').firstChild.data;
}
This may seem like a bit to work with at first, but with a little
work, you can get what you want to work. If you want to avoid this,
you could use the responseText method, parsing the XML in PHP before
sending it to the XMLHttpRequest object.
Quick Tip 3: (Relevant Links):
* Microsoft XMLHttpRequest Documentation
* XMLHttpRequest @ Apple Developer Connection
* The DOM Model in Mozilla
About the Author:
James Kassemi lives in Albuquerque, New Mexico. He works mainly with
PHP, programming for a variety of clients in the southwest.
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