Is it a good idea ? (not tested)

backend.php

<?php 

// Alternative to the long polling system used in lichess (github.com/ornicar/lichess)
// - We could use an ajax request on a php script with an infinite loop until version has changed.

$userVersion = $_GET['version']
$version = $db->stack->find($_GET['player_id'])->getVersion();

while ($userVersion <= $version) {
     usleep(10000);
     $version = $db->stack->find($_GET['player_id'])->getVersion();
     if ($version !== $userVersion) {
        return json_encode(array(
            'events' => $stack->getEventsSince($userVersion))),
            'version' => $version,
        );
     }
}