Introduction to several PHP callback functions

$server->on('Request', function ($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'));