For Julian

AbstractCello.php

<?php

abstract class AbstractCello
{
    /**
     * @var EventManager
     */
    protected $em;
    
    protected function triggerPre($methodName, $params)
    {
        $e = new Event($methodName.'.pre', $this, $params);
        $result = $this->em->trigger($e, function($o){ return $o !== null; });
        if(($lastResult = $result->last()) !== null){
            return $lastResult;
        }
    }
    
    // ... em getters and setters
}

Cello.php

<?php
use Zend\EventManager\Event

class Cello extends AbstractCello
{
    public function foo($a, $b = 15, $c = 'bar')
    {
        if($result = $this->triggerPre(__FUNCTION__,[
            'a' => &$a,    // we're using references to be able
            'b' => &$b,    // to modify call params 
            'c' => &$c     // on the spot... 
        ])){
            return $result;
        }
        
        // expensive calls
        return $something;
    }
}

usecase.php

<?php

use Zend\EventManager\Event;

$cello = new Cello();
$cello->getEvents()->attach('foo.pre',function(Event $e){
    if($event->getParam('a') == 'foo'){
        
        // override the value of param b
        $event->getParam('b') == 1024;
        
    }elseif($event->getParam('a') == 'bar'){
        
        // short-circuit execution of foo() method
        return 'some result';
    } 
});


var_dump($cello->foo('foo', 20)); // this will always have the second param === 1024
var_dump($cello->foo('bar'));     // this should always return "some result"