如何在不实例化类的情况下使用typehinting获取对象的数据

Question

I have seen this example in frameworks like silex, symfony and I am not sure how it is done.

basically, I would like to get the content id Request without instantiating it hiding the instantiation part and simply call the class via a functions argument.

Here is a basic example.

<?php 

class Request {
    function get($key){
        $_GET[$key] = 42; 
        return $_GET[$key]; 
    }
}

$route = function(Request $request){
    // how to '42' without instanciating Request here
    var_dump($request->get('..'));
};

var_dump($route);