HTML5 SSE

### What is Server-Sent Event?
HTML5 server-sent event is a new way for the web pages to communicating with the web server. It is also possible with the XMLHttpRequest object that lets your JavaScript code make a request to the web server, but it's a one-for-one exchange — that means once the web server provides its response, the communication is over. XMLHttpRequest object is the core of all Ajax operations.

However, there are some situations where web pages require a longer-term connection to the web server. A typical example is stock quotes on finance websites where price updated automatically. Another example is a news ticker running on various media websites.

You can create such things with the HTML5 server-sent events. It allows a web page to hold an open connection to a web server so that the web server can send a new response automatically at any time, there's no need to reconnect, and run the same server script from scratch over and over again.

> Note: Server-Sent Events (SSE) are unidirectional that means data are delivered in one direction from the server to client. A client typically is a web browser.

> Tip: The HTML5's server-sent events feature is supported in all major modern web browsers like Firefox, Chrome, Safari and Opera except Internet Explorer.

### Sending Messages with a Server Script
Let's create a PHP file named "server_time.php" and type the following script into it. It will simply report the current time of the web server's built-in clock in regular intervals. We will retrieve this time and update the web page accordingly later in this tutorial.

```php
<?php
header("Content-Type: text/event-stream");
header("Cache-Control: no-cache");
 
// Get the current time on server
$currentTime = date("h:i:s", time());
 
// Send it in a message
echo "data: " . $currentTime . "\n\n";
flush();
?>
```
The first two line of the PHP script sets two important headers. First, it sets the MIME type to `text/event-stream`, which is required by the server-side event standard. The second line tells the web server to turn off caching otherwise the output of your script may be cached.

Every message send through HTML5 server-sent events must start with the text `data:` followed by the actual message text and the new line character sequence (`\n\n`).

And finally, we have used the PHP `flush()` function to make sure that the data is sent right away, rather than buffered until the PHP code is complete.

### Processing Messages in a Web Page
The `EventSource` object is used to receive server-sent event messages.

Now let's create an HTML document named "demo_sse.html" and place it in the same project directory where the "server_time.php" file is located. This HTML document simply receives the current time reported by the web server and display it to the user.

```html
<!DOCTYPE html>
<html lang="en">
<head>
<title>Using Server-Sent Events</title>
<script>
    window.onload = function() {
        var source = new EventSource("server_time.php");
        source.onmessage = function(event) {
            document.getElementById("result").innerHTML += "New time received from web server: " + event.data + "<br>";
        };
    };
</script>
</head>
<body>
    <div id="result">
        
    </div>
</body>
</html>
```