header('WWW-Authenticate: Basic realm="my realm"');
    header('HTTP/1.0 401 Unauthorized');
    echo "You need to enter a valid username and password.";
    exit;

if (!isset($_SERVER['PHP_AUTH_USER'])) {
    header('WWW-Authenticate: Basic realm="My Realm"');
    header('HTTP/1.0 401 Unauthorized');
    echo 'Текст, отправляемый в том случае,
    если пользователь нажал кнопку Cancel';
    exit;
}

    unset($_SERVER['PHP_AUTH_USER']);
   
   if (!isset($_SERVER['PHP_AUTH_USER'])) { 
    header('WWW-Authenticate: Basic realm="My Realm"');
    header('HTTP/1.0 401 Unauthorized');
    echo "You need to enter a valid username and password.";
    exit;
   }

    if (!isset($_SESSION['reauth']))
    {
        
        header('WWW-Authenticate: Basic Realm="Login please"');
        header('HTTP/1.0 401 Unauthorized');
        $_SESSION['auth'] = true;
        $_SESSION['reauth'] = true;
        echo "Необходимо авторизоваться";
        exit;
    }
    $_SESSION['reauth'] = null;