<p id="value">The value is 1 now.</p>
<button id="check">Check Value</button>

var button = document.getElementById('check');
//Trigger button click.
button.addEventListener('click', function(){
    //Check if EventSource is supported.
    if(typeof(EventSource)!="undefined")
    {
        //EventSource to 'index.php'
        var evtSource = new EventSource('index.php');
        //Trigger receiving a massage.
        evtSource.addEventListener("msg", function(e) {
            var obj = JSON.parse(e.data);
            //Check if the value is 2.
            if( obj == '2' ){
                //Change the paragraph.
                paragraph.innerHTML = 'The value is 2 now.';
                //Close EventSource.
                evtSource.close();
            }
        }, false);
    }
});

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

//Connect to the DB.
$servername = "localhost";
$username = "root";
$password = "";
$dbname = "value";
try {
    $conn = new PDO("mysql:host=$servername;dbname=$dbname", $username, $password);
    // set the PDO error mode to exception
    $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
}catch(PDOException $e){
    echo "Error: " . $e->getMessage();
}

//Fetch the value.
$val = $conn->prepare('SELECT v FROM val');
$val->execute();
$val = $val->fetchAll();
$val = $val[0]['v'];

//Check the value
if ($val == '2') {
    echo "event: msgn";
    echo 'data: "2"';
    echo "nn";

}

ob_flush();
flush();