Requirement

composer require kmaestro/gtk

sudo apt-get install libgtk-3-dev

require __DIR__ . '/vendor/autoload.php';
use Gtk3\Gtk;
use Gtk3\Gtk\Window;
use Gtk3\Enum\WindowEnum;
$gtk = Gtk::getInstance();
$gtk->init();
$window = new Window(WindowEnum::topLevel);
$window->widget()->show();
$gtk->main();
return 0;