PHP SSE , data not shown properly

i tried to create my own SSE using php and js, but don’t receive data each second instead,
get all data after 1min , OR my browser create each 5 second new request :

PHP : (https://github.com/mdn/dom-examples/blob/main/server-sent-events/index.html)

        header("X-Accel-Buffering: no");
        header("Content-Type: text/event-stream");
        header("Cache-Control: no-cache");

        $counter = rand(1, 10); // a random counter
        while (1) {
            // 1 is always true, so repeat the while loop forever (aka event-loop)

            $curDate = date(DATE_ISO8601);
            echo "event: pingn",
            'data: {"time": "' . $curDate . '"}', "nn";

            // Send a simple message at random intervals.

            $counter--;

            if (!$counter) {
                echo 'data: This is a message at time ' . $curDate, "nn";
                $counter = rand(1, 10); // reset random counter
            }

            // flush the output buffer and send echoed messages to the browser

            while (ob_get_level() > 0) {
                ob_end_flush();
            }
            flush();

            // break the loop if the client aborted the connection (closed the page)

            if (connection_aborted()) break;

            // sleep for 1 second before running the loop again

            sleep(1);
        }

JS :

var source = new EventSource("sse2");
  source.onmessage = function (event) {
    console.log(event.data);
  };

2 Results :

any advise to fix that.