[
    { "firstName": "John", "lastName": "Smith", "login": "john", "password": "qwerty" },
    { "firstName": "Stella", "lastName": "Hoe", "login": "stella", "password": "asd123 "}
]

session_start();                                                                                                                                                                    [1/7]

$users = json_decode(file_get_contents(__DIR__ . '/users.json'), true);

$method = $_SERVER['REQUEST_METHOD'];
if ($method === 'POST' && @$_POST['action'] === 'login') {
    $login = $_POST['login'];
    $password = $_POST['password'];

    $user = findUser($login, $password, $users);

    if ($user) {
        $_SESSION['logged_user'] = $user;
        die(json_encode($user));
    } else {
        $_SESSION['logged_user'] = null;
        die('{"error": "Incorrect login or password"}');
    }
} elseif ($method === 'POST' && @$_POST['action'] === 'logout') {
    $_SESSION['logged_user'] = null;
    die('{}');
} elseif ($method === 'GET') {
    $user = @$_SESSION['logged_user'];
    if ($user) {
        die(json_encode($user));
    } else {
        die('{}');
    }
}

function findUser($login, $password, $users) {
    $filtered = array_filter($users, function($user) use ($login, $password) {
        return $user['login'] === $login && $user['password'] === $password;
    });

    return @$filtered[0];
}