SimpleDelegate.php

<?php

class SimpleDelegate {

    protected $callbacks = [];

    public function add(Closure $callback, $key = null){
        $key = $key?: spl_object_hash($callback);
        $this->callbacks[$key] = $callback;
    }

    public function remove(Closure $callback){
        $key = spl_object_hash($callback);
        unset($this->callbacks[$key]);
    }

    public function removeByKey($key){
        unset($this->callbacks[$key]);
    }

    public function __invoke() {
        $results = [];
        foreach($this->callbacks as $callback){
            $results[] = call_user_func_array($callback, func_get_args());
        }
        return $results;
    }
}

class Foo{

    private $delegate;

    private $counter = 0;
    private $text1 = '';
    private $text2 = '';
    private $text3 = '';

    public function __construct(){
        $this->delegate = new SimpleDelegate;
        $this->delegate->add(function($arg){
            $this->privateMethod($arg);
        });
        $callback2 = function($arg){
            $this->privateMethod2($arg);
        };
        $callback3 = function($arg){
            $this->privateMethod3($arg);
        };
        $this->delegate->add($callback2);
        $this->delegate->add($callback3);
        $this->delegate->remove($callback2);

        $this->delegate->__invoke('abc.');
        $this->delegate->__invoke('ABC.');
        $this->delegate->__invoke('AAA.');
    }

    private function privateMethod($arg){
        $this->counter++;
        $this->text1 .= $arg;
    }

    private function privateMethod2($arg){
        $this->text2 .= $arg;
    }

    private function privateMethod3($arg){
        $this->text3 .= $arg;
    }
}

var_dump(new Foo);