Pushing Updates to the Web Page with HTML5 Server-Sent Events

The HTML5 Server-Sent event model allows you to push realtime data updates from the server to the browser.

In this tutorial we will work through the process, with the EventSource object handling received data and writing it to the page. We will use HTML5 and JavaScript at client side, with PHP at server side.

With existing models such as AJAX, the code in a Web page would continually ask the server to supply new data, but the onus was on the client to request the information. With Server-Sent requests, you can stream data from your server, with updates pushed from there without the code at client side having to keep requesting it. Once your page initiates the Server-Sent event, the server script can continue sending updates. Your JavaScript code can write this new data into the page whenever it receives it.

Create an HTML5 Page

Create your HTML5 page as follows:

<!DOCTYPE html>

Add an Element to Display the Server-Sent Data

We will display the data received from the server in a dedicated page element, so add the following in the body section of your page:

<div id="serverData">Here is where the server sent data will appear</div>

You can put any content you like in your element, as long as it has an ID attribute so that you can identify it in your script. The placeholder text should only appear when the page first loads, but will disappear when the script runs.

Add a Script to the Page

Because we want the function to continue receiving and handling updates, we add the script section in the body of the page. Of course your own pages may execute functions on user interaction, but for the purposes of this demonstration add the following script at the end of your page, before the closing body tag:

<script type="text/javascript">
//functions here

The script will execute when the browser renders the page, so the Server-Sent events will be initiated straight away. Next add the JavaScript code inside this script section:

//check for browser support
if(typeof(EventSource)!=="undefined") {
	//create an object, passing it the name and location of the server side script
	var eSource = new EventSource("send_sse.php");
	//detect message receipt
	eSource.onmessage = function(event) {
		//write the received data to the page
		document.getElementById("serverData").innerHTML = event.data;
else {
	document.getElementById("serverData").innerHTML="Whoops! Your browser doesn't receive server-sent events.";

The script first checks that the browser supports the EventSource model. If it doesn’t, an error message is written to the server data page element instead. If the browser does support the function, the Server-Sent process begins. First, the script creates an object of the EventSource class, passing it the URL of the server side script that will be providing the streamed data updates. Then the script sets up an event listener function to execute when the EventSource object receives an update from the server. When this happens, the script simply gets a reference to the update page element and writes the new data to it.

Create a Server Side Script

We now need to create the server side PHP script to send updates to the page. Create a new file and save it “send_sse.php” or another name of your choice, as long as you amend the JavaScript EventSource code above to reflect the correct name and location. Enter the following outline in your PHP file:

//streaming code

Begin your PHP script by setting the headers as follows:

header('Content-Type: text/event-stream');
header('Cache-Control: no-cache');

This code sets the script up to stream data, preventing caching issues. Add the following inside the script, before the closing PHP tag:

//generate random number for demonstration
$new_data = rand(0, 1000);
//echo the new number
echo "data: New random number: $new_data";

For the purposes of demonstrating Server-Sent events, we are simply generating a new random number each time the script sends an update. The code first stores a new random number between zero and one thousand as a variable. Then it echoes the new data along with some text, after the text “data: ” which you must include for your script to function correctly. Finally the script flushes the output, sending it to the browser.

Upload and Test

You will need to run these scripts on a server to see them functioning correctly. Upload your HTML page and PHP script to the same directory. Browse to the page and watch the updates appear one after another.


In this tutorial we have carried out the basics of using Server-Sent events with HTML5, JavaScript and PHP. In our example server side script we simply generated random numbers, but in your own projects you can carry out other server side processes such as fetching data from different sources, sending fresh information to your users without their browser having to request these updates each time.

Check out this tutorial in action here: Pushing Updates to the Web Page Demo

Sue Smith works as a Web/ software developer and technical writer based in the UK: see benormal.info for details. Sue has written for various clients including Smashing Magazine and Mobiletuts+. She also does a little Android development and some comedy writing. More articles by Sue Smith
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