Server Side Events Start/Stop

I am displaying some real time data from a MySQL database using PHP and JavaScript. Everything works as expected and the data is displaying as I want.

However, I would now like to implement two simple buttons to achieve the following;

• stop - pause the current real time feed
• start - resume the feed from where it was paused

I can get the stop button to work by using the EventSource close() function. I'm not so sure about how to resume from where I left off though. I think I may need to include the Last-Event-ID as described here.

My code so far is;

PHP

<?php 
session_start(); 
include 'conn.php'; // database connection
header("Content-Type: text/event-stream");
header("Cache-Control: no-cache");
header("Access-Control-Allow-Origin: *");

$query = "SELECT TimeStamp, CardNo, SerialNo FROM transactions ORDER BY TimeStamp DESC LIMIT 1";
$stmt = $conn->prepare($query);
$stmt->execute();
$result = $stmt->get_result();

if($row = $result->fetch_assoc()) {
    echo "retry: 100\n\n"; 
    if ($_SESSION["lastSerialNo"] != $row["SerialNo"]) {
        $_SESSION["lastSerialNo"] = $row["SerialNo"];
        echo "data: ".$row['SerialNo']. ' ' .$row['TimeStamp']. "\n\n";
        flush();
    }
    else {
        // do nothing
    }
}

JS & HTML

<script type="text/javascript">
    var source = new EventSource("data.php");
    window.onload = function() {
        source.onmessage = function(event){
            document.getElementById("result").innerHTML += "Data : " + event.data + "<br>";
        };
    };

    function stop() {
        source.close(); // this successfully stops the feed
        };

    function start() {
        // how to resume the feed?
        };  

    </script>

<button id="stop" onclick="stop()"> stop</button>
<button id="start" onclick="start()"> start</button>

Any help is appreciated.