If you’ve ever had to work with XML from another domain, and only had JavaScript in your toolbox, you can feel my pain. Cross-domain XML has been the bane of my existence for many years now, when working on sites where there is no server-side scripting language available. jQuery has some great AJAX features, but you can’t make use of any of them if the file you’re trying to retrieve is an XML file on another domain. Until now. PHP is still required as part of this equation, but it acts as a web service, rather than residing on the server where your site lives.
Setting Up PHP as a Web Service
Thanks to a few new JSON functions added in PHP 5.2, we can build an XML to JSON web service in 4 lines of code. That’s right, 4 lines. Here it is, in all its glory:
$c = $_GET['car'];
$s = file_get_contents('http://www.example.com/' . $c . '.xml');
$a = json_decode(json_encode((array) simplexml_load_string($s)),1);
echo $_GET['callback'] . '(' . json_encode($a) . ');';
This code would reside on a server separate from the server where your site is hosted. And now the break down:
$c = $_GET['car'];
Get the value from the query string (browser’s url bar) with the name “car”, which we’ll see when we build the jQuery request, and save it in a variable named “$c”. Easy enough.
$s = file_get_contents('http://www.example.com/' . $c . '.xml');
Get the contents of a remote XML file, inserting the $car variable in the URL, and save in a variable named “$s” (for “string”).
$a = json_decode(json_encode((array) simplexml_load_string($s)),1);
Convert the XML string into a JavaScript array using PHP’s new json_decode and json_encode functions, and save this new array in a variable titled “$a”.
echo $_GET['callback'] . '(' . json_encode($a) . ');';
Here’s where the magic happens. We take that array, and convert it into JSON using PHP’s json_encode function. We also get the “callback” variable from the query string, which is required by jQuery, and append parentheses around the encoded JSON. The appended parentheses are also known as the “P” in JSONP, or “JSON with Padding”. What this does is essentially create a JavaScript function to pass back to your site that looks like this:
callback987986215(allyourJSONdata)
Setting Up jQuery to Call Your Web Service
Once you’ve got your PHP web service setup to convert XML to JSON, making the request in jQuery is pretty simple as well. Here’s how its done:
$(document).ready(function() {
$.ajax({
dataType: 'json',
url: 'http://www.example2.com/my-web-service.php?car=mercedes&callback=?',
success: function(data) {
//parse your new JSON object here
}
});
});
This is a pretty standard jQuery AJAX request. The url paramater is the most important piece here. We’re looking for the JSON data on our server where our PHP service is hosted, and passing 2 variables:
- car with a value of mercedes, and
- callback with a value of ?
The variable of car=mercedes depends on the site where you’re pulling your XML from. This isn’t required, it just demonstrates how you could build a flexible web service to leverage for more than one external XML feed. The second variable callback=? is required by jQuery. The question mark is just there to append an additional name/value pair. jQuery will recognize the variable callback=? and replace the question mark with a randomly generated string of numbers. That’s why in your PHP web service, you get the callback variable, and send it back to jQuery as the function name. Its a sort of security measure.
That’s really it; from this point you have a jQuery object to work with containing all the data in the original XML file. You can see the jQuery object and all its pieces easily by setting a few breakpoints in Firebug and digging in.
If you want to get into the gritty details of how this works, like the coolness of how you can call functions in JavaScript by passing JSON as the argument, and how PHP can convert XML to an array and then to JSON, check out a few of the articles I referenced while building this:
I read Simply JavaScript a few months back, and couldn’t help but include it in my reviews here at withinsight.com. Its simply too good not to. I’ve got a decent amount of JavaScript experience, although not necessarily through practice. JavaScript has always been that part of my web design arsenal that I’ve wanted desperately to add, but has never seemed to work its way into regular usage in my day-to-day work. You can’t say its for lack of trying, as I’ve read the first half of O’Reilly’s JavaScript, The Definitive Guide, which while full of great info, is not necessarily the best introduction to JavaScript for the beginning scripter. I then found DOM Scripting by Jeremy Keith, which offers a very, very introductory level explanation of JavaScript before digging into the basics of DOM Scripting. I had a decent picture of what else was out there in terms of JavaScript books.
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