服务器发送的事件未推送到目的地

I'm trying to push new contents for a simple div element on a web page (content consists of 2 iFrames). I've followed the online tutorials closely.

The setup is:

• a "switcher" web page with a couple of buttons to select different contents. Clicking a button opens a PHP file with the button number passed in the $_REQUEST.
• the PHP script that fetches the content from a database and sends the event
• the target html page that I want to display the div. It has a JavaScript that sets up a listener to the PHP file.

When the switcher button invokes the PHP file, the PHP file opens in a browser window and displays the text that I want the target window to receive. The listener in the target page doesn't appear to be getting the message. (I have the text/event-stream and no-cache headers defined.)

This is a very minimal app and closely follows the tutorials. The pages are being served by apache2 and PHP 7 on a Raspberry Pi. I have the feeling I'm missing something simple.

In index.html, which is supposed to replace the contents of a div with the received text:

<div id="serverData">  ...(2 iframes)... </div>
... 
<script type="text/javascript">
        //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("ViewController.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.";
        }
    </script>

In ViewController.php, the source of the update:

<?php
header('Content-Type: text/event-stream');
header('Cache-Control: no-cache');
//stream new webpages to the main view

$myPDO = new PDO('sqlite:/home/pi/pi_DB');
if (isset($_REQUEST['pageNo']) ) {
    $result = $myPDO->query("SELECT htmlContent FROM views WHERE pageID = " . $_REQUEST['pageNo']);

    $queryResult = $result->fetch(PDO::FETCH_ASSOC);
    echo "event: message
";  // same result with or without this line
    echo "data: " . $queryResult['htmlContent'] . "

";
    flush();
}

In viewswitcher.html which has the buttons to select different contents:

<p>
    <button class="button" onclick="swapViewIntoIndex(2)">Select View 2</button>
</p>
<script>
    function swapViewIntoIndex(viewNumber) {
        // load ViewController.php and pass it the button (view) number
        switch (viewNumber) {
            case 1:
                var url = 'ViewController.php?pageNo=1';
                window.location.href = url;
                break;
            case 2:
                var url = 'ViewController.php?pageNo=2';
                window.location.href = url;
                break;
            default:
        }
    }