Today i want to introduce you to jSON (JavaScript Object Notation), in short, it is a simple format designed to exchange data between different programming languages. I will show you how to create JavaScript object, convert it to JSON string, and send to PHP script, which will decode jSON string to readable format (for PHP). But that’s not all, PHP script will create it’s own data object encode it to jSON string and send it back. All communication between JavaScript and PHP will be done thru AJAX.

If you haven’t heared about jSON yet, then you can visit Wikipedia for more information. The other technology you need to be familiar with before reading this article is AJAX. If you need to, you can read my Introduction to AJAX post.

JSON objects

Usually when it comes to JSON we have an encoded string in mind, however JSON is a subset of JavaScript and in this programming language it can be used as is, to create objects. Simple JavaScript object created using JSON notation can look like this:

var JSONstring = 
{
    "firstname": "Greg", 
    "email": "greg@fake_email.com",
    "hobby": 
    [
	{
	    "hobbyName": "sport", 
            "isHobby": "true"
	},
        {
		"hobbyName": "reading", 
	        "isHobby": "true"
	},
        {
		"hobbyName": "music", 
	        "isHobby": "false"
	}
    ]
};

Accessing fields is done like in any other JS object (mainly because it is “normal” JavaScript object), if we want to know if hobby “reading” was checked then we would have to write:

JSONstring.hobby[1].isHobby; // true

Creating JavaScript Objects

Before we start, we will need something to work with. We will create HTML form with “validate” button, when someone clicks this button the whole proccess i described in the first paragraph will start. Also, despite JSON is a subset of JavaScript there are no built in function for converting JavaScript object into JSON string, so we will use already created class available at JSON homepage, the file is located here json2.js.

Here is the code for the HTML form, i think there is no need to explain it:

</script>
<html>
<head><TITLE>ditio.net jSon Tutorial</TITLE>
<script src="http://www.json.org/json2.js"></script>
<script>
// JavaScript source code will be here
</head>
<body>
<form name="personal" action="" method="POST">
Name <input type="text" name="firstname"><br>
Email <input type="text" name="email"><br>
Hobby 
	<input type="checkbox" name="hobby" value="sport"> Sport
	<input type="checkbox" name="hobby" value="reading"> Reading
	<input type="checkbox" name="hobby" value="music"> Music
<input type="button" name="valid" value="Validate" onclick="validate()">
</form>
</body>
</html>

After clicking “validate” button validate() function will be called, we need to add it, in head section for example:

function validate()
{
    var p = document.forms['personal'];
 
    var JSONObject = new Object;
    JSONObject.firstname = p['firstname'].value;
    JSONObject.email = p['email'].value;
    JSONObject.hobby = new Array;
 
    for(var i=0; i<3; i++)
    {
        JSONObject.hobby[i] = new Object;
	JSONObject.hobby[i].hobbyName = p['hobby'][i].value;
	JSONObject.hobby[i].isHobby = p['hobby'][i].checked;
    }
 
    JSONstring = JSON.stringify(JSONObject);
    runAjax(JSONstring);
 
}

Code is quite easy to understand. First i assign whole form to variable p just to make further access to this form data easier. In next lines JavaScript object is created, as you can see it consists only from Object and Array data objects.

Note that this is completely the same object as in first listing of this tutorial, however different method was used to create it.

Sending JSON object to PHP with AJAX

I do not want ot get into AJAX here because it is not the topic of this tutorial, i will use code from my Introduction to AJAX post, if you do not want ot go there then remember that at the end of this article there is whole source code waiting to be downloaded.

var request;
function runAjax(JSONstring)
{
    // function returns "AJAX" object, depending on web browser
    // this is not native JS function!
    request = getHTTPObject();
    request.onreadystatechange = sendData;
    request.open("GET", "parser.php?json="+JSONstring, true);
    request.send(null);
}
 
// function is executed when var request state changes
function sendData()
{
    // if request object received response
    if(request.readyState == 4)
    {
	// parser.php response
	var JSONtext = request.responseText;
	// convert received string to JavaScript object
	var JSONobject = JSON.parse(JSONtext);
 
	// notice how variables are used
	var msg = "Number of errors: "+JSONobject.errorsNum+
		"\n- "+JSONobject.error[0]+
		"\n- "+JSONobject.error[1];
 
	alert(msg);
    }
}

That’s it we are half way there, now we need to create PHP script to handle AJAX request.

JSON and PHP

Decoding JSON string is very simple with PHP only one line of code is needed to parse string into object. Similary only one function is needed to encode PHP object or array into JSON string, look at the code:

<?php
 
// decode JSON string to PHP object
$decoded = json_decode($_GET['json']);
 
// do something with data here
 
// create response object
$json = array();
$json['errorsNum'] = 2;
$json['error'] = array();
$json['error'][] = 'Wrong email!';
$json['error'][] = 'Wrong hobby!';
 
// encode array $json to JSON string
$encoded = json_encode($json);
 
// send response back to index.html
// and end script execution
die($encoded);
 
?>

It is also interesting what is inside $decode variable

stdClass Object
(
    [firstname] => fgfg
    [email] => 
    [hobby] => Array
        (
            [0] => stdClass Object
                (
                    [hobbyName] => sport
                    [isHobby] => 1
                )
 
            [1] => stdClass Object
                (
                    [hobbyName] => reading
                    [isHobby] => 
                )
 
            [2] => stdClass Object
                (
                    [hobbyName] => music
                    [isHobby] => 
                )
 
        )
)

PHP finished execution, then “request” object status in JavaScript is now equal to 4. Response text (in sendData() function) will be parsed with JSON class to object and used to display message on the screen. Note that instead of using JSON.parse() we could use JavaScriipt eval() function.

Conclusion

This tutorial was intended to introduce you to JSON, and i wanted to make this tutorial as clear as possible so i intentionally used the simplest methods to achieve my goal. However this tutorial wouldn’t be complete if i wouldn’t give you further resources from which you can learn more.

First you should check out is Zend_Json class (a part of Zend Framework), it has the same functionality as json_decode() and json_decode(), but can handle more complicated JSON strings then those two functions.

Second is json.org home of JSON, check especially this tutorial, it has got great examples of more advanced JSON class usage.