$server->on('Request', function ($req, $resp) {
    echo "hello world";
});

class A
{
    static function test($req, $resp)
    {
        echo "hello world";
    }
}
$server->on('Request', 'A::Test');
$server->on('Request', array('A', 'Test'));

function my_onRequest($req, $resp)
{
    echo "hello world";
}
$server->on('Request', 'my_onRequest');

class A
{
    function test($req, $resp)
    {
        echo "hello world";
    }
}

$object = new A();
$server->on('Request', array($object, 'test'));