client.html

<script src="event-source.js"></script>

event-source.js

document.addEventListener('DOMContentLoaded', function () {
  
  var eventSrc  = new EventSource('events.php');
  
  eventSrc.addEventListener('open', function (event) {
    console.log(event.type);
  });
  
  eventSrc.addEventListener('message', function (event) {
    
    
    console.log(event.type);
    console.log(event.data);
  
  });


}, false);

events.php

<?php
header("Content-Type: text/event-stream\n\n");

// despite not having the while(true){}
// this seems to repeat pushing messages to the client
echo 'data: ' . time() . "\n";
?>

The reason it repeats is that the connection is closed after the script terminates. The EventSource tries to reconnect, gets one message and the connection is terminated again. Repeat. Not really SSE but polling... looping and holding the connection open is the point of SSE, as it removes the HTTP overhead of the polling.