nginx, PHP, session data trouble with server sent event

I'm using several technologies for a web application:
nodejs for socket.io and request
nginx / PHP
server sent event
session store in a mongodb collection.

• node/socketio/request: acts as a middleware, in place of the "old" ajax way. adding "set-cookie" in the posted header makes things work really fast, keeping session data across navigation.
• Sessions are stored in a mongodb base, using a session_save_handler.
• A server sent event connection is made, and here is the strange behavior:

if I don't put 'session_write_close' in top of the called file of sse, then session is lost on page refresh.
If I put it, ok seems fine, but I can't monitor any changes of the session object, which does not vary, even on user login/logout.

Here is my session Class file:

class Session {
    protected $dbSession;
    protected $maxTime;

    public function __construct() {
        $this->conn =  new Mongo('mongodb://user:passwd@hostip');
        $sitebase_app   = 'base_for_coll';

        $this->dbSession     = $this->conn->$sitebase_app->session;
        $this->maxTime = get_cfg_var("session.gc_maxlifetime");

        register_shutdown_function('session_write_close');
        session_set_save_handler(
            array($this, 'open'),
            array($this, 'close'),
            array($this, 'read'),
            array($this, 'write'),
            array($this, 'destroy'),
            array($this, 'gc')
        );

    }
    public function open() { return true; }
    public function close() { return true; }
    public function read($id) {
        $doc = $this->dbSession->findOne(array("_id" => $id), array("sessionData" => 1));
        return $doc['sessionData'];
    }
    public function write($id,$data) {
        //
        $this->dbSession->save(array("_id" => $id, "sessionData" => $data, "timeStamp" => time()));
        return true;
    }
    public function destroy($id) {
        $this->dbSession->remove(array("_id" => $id));
        return true;
    }
    public function gc() {
        $agedTime = time() - $this->maxTime;
        $this->dbSession->remove(array("timeStamp" => array('$lt' => $agedTime)));
    }
}

ini_set('session.save_handler' , 'user');
$session = new Session();

And voila my SSE php file:

include_once($_SERVER['CONF_INC']);

$APP = new App();

$APPCACHE = $APP->plug('sitebase_sockets','data_activity');
$APPCACHE->ensureIndex(['timeData_activity'=>1]);
$APPCACHE->ensureIndex(['codeData_activity'=>1]);

ini_set('output_buffering', 'off');
ini_set('zlib.output_compression', false);
header('Content-Type: text/event-stream');
header('Cache-Control: no-cache');
header('Content-Encoding: none;');

session_write_close();

flush();
ob_end_flush();
//
$time = time();
$lastmodif = filemtime(ACTIVEMODULEFILE);



function sendMsg($id , $msg) {
    ini_set('implicit_flush', true);
    ob_implicit_flush(true);
    $sessid = session_id();
    echo "id: ".uniqid() . PHP_EOL;
    echo "event: $id" . PHP_EOL;
    echo "data: {\n";
    echo "data: \"msg\": $msg  \n";
    echo "data: }\n";
    echo PHP_EOL;
    ob_flush();
    flush();
}


$notify = 1;

 


$startedAt = time();
do {

    $TEST = $APPCACHE->find(['timeData_activity'=>['$gte'=>$startedAt]]);
    if($TEST->count()!=0){
        // event : act_upd_data
        // data : json
        $startedAt = time();
         
        while($ARR = $TEST->getNext()){
            unset($ARR['_id']);
            $json = json_encode($ARR,JSON_FORCE_OBJECT);
            sendMsg($ARR['eventData_activity'] ,$json);
        }
        sleep(1);
    }
    if ((time() - $startedAt) > 360) {
        skelMdl::send_cmd('act_notify',['options'=>['sticky'=>'true']]+ array('msg'=>'DIE FOR PERIODIC TEST RELOAD'.session_id()),session_id());
        die();
    }

    usleep(250000); // 1/4 seconde

    ob_flush();
    flush();
    
    if(empty($_SESSION['idagent'])){
        $notify = 0;
        skelMdl::send_cmd('act_notify', array('msg'=>' Session agent empty'.session_id()),session_id());
        sleep(5);
    }

} while(true);

And the start of my conf.inc.php file

header('Content-Type: text/html; charset=UTF-8');
//
ini_set('display_errors', 55);
// THE SESSION_HANDLER
include_once('appclasses/ClassSession.php');
// 

if ( session_status() == PHP_SESSION_NONE ) { 
    session_start();
}

I cannot find any more ideas, any advice would be appreciated. Thanks

p.s.: socket.io instead of nature websocket, because of rooms and namespace... But of course, I would prefer using native websocket, avoiding the use of a middleware...