I want to set the session in my service file:

<?php session_start();

require_once '../entities/User.php';
require '../vendor/autoload.php';
use MongoDBClient as MongoClient;
class TestService{
    private static $client = null;

    private static function getClient() {
        if (self::$client === null) {
            
            self::$client = new MongoClient('mongodb://localhost:27017'); 
        }
        return self::$client;
    }

    public static function saveMessage($message,$date) {
        $client = self::getClient();
        $collection = $client->forumTest->test;
        
        $result = $collection->insertOne(['message' => $message, 'date' => $date]);
        return $result->getInsertedCount() === 1;
    }

    public static function checkUsername($username){
        $client = self::getClient();
        $collection = $client->forumTest->users;
        
        $user = $collection->findOne(['username' => $username]);
        return $user !== null;
    }

    public static function saveUser(User $user){
        $client = self::getClient();
        $collection = $client->forumTest->users;

        $result = $collection->insertOne(['username' => $user->getUsername(),'password' => $user->getPassword()]);
        return $result->getInsertedCount() === 1;
    }

    public static function checkUser(User $user){
        $client = self::getClient();
        $collection = $client->forumTest->users;

        $result = $collection->findOne(['username' => $user->getUsername(),'password' => $user->getPassword()]);
        if($result !==null){
            $username = $user->getUsername();
            $_SESSION['username'] = $username;
        }
        return $result !== null;
    }
}
?>     

And I want to call it here:

<?php session_start();

header("Access-Control-Allow-Origin: http://localhost:3000");
header("Access-Control-Allow-Methods: GET, POST, OPTIONS");
header("Access-Control-Allow-Headers: Content-Type");
header("Access-Control-Allow-Credentials: true");


$loggedIn = isset($_SESSION['username']);
echo json_encode(['loggedIn' => $loggedIn]);
?>

But the $_SESSION[‘username’] is null. I tried to do a test session with two test files and it worked so I guess the problem is in my code.

testFile1:

<?php session_start();

$asd = "asd";
$_SESSION['asd'] = $asd;

?>

TestFile 2:

<?php session_start();
echo $_SESSION['asd'];
?>

It works like this, but when I want to use it in my Service and call it in another file it doesn’t work.