Description:
------------
It should be useful to encapsulate PHP pseudotype callback using new magic method __invoke(). But this type of "callable" objects are not distinguishable by any interface or class name (compare it with ArrayAccess or Countable interfaces).

// this is simple callback encapsulation
class Callback 
{
	private $callback;

	public function __construct($callback)
	{
		$this->callback = $callback;
	}

	public function __invoke()
	{
		$args = func_get_args();
		return call_user_func_array($this->callback, $args);
	}
}


// a Callback object:
$callback = new Callback(array('MyClass', 'anyMethod'));

// a Closure object:
$closure = function() { ... };


The problem: there is nothing common between Callback and Closure object, what can be used (for example) as type hinting. 

First solution using interface:

interface Callable { 
    function __invoke()
}

class Callback implements Callable { ... }

function event(Callable $obj)
{
	$obj($sender, $param);
}

event($callback);
event($closure); // do not work

The class Closure is not Callable implementor thus it do not work. Problematic is __invoke method's parameters too.

The other way:

class Callback extends Closure { ... } // do not work

function event(Closure $obj)
{
	$obj($sender, $param);
}

event($callback);
event($closure);

This is not possible, because Closure is final. Solution is make Closure non-final.

The third way - create static factory to encapsulate PHP callback as Closure:

function event(Closure $obj)
{
	$obj($sender, $param);
}

$callback = Closure::factory(array('MyClass', 'anyMethod'));
event($callback);
event($closure);

Maybe the most easy way.