CallbackExecutor.php

class CallbackExecutor 
{    
    protected $callback;

    public function __call($method, $args)
    {
        if(is_callable(array($this, $method))) {
            return call_user_func_array($this->$method, $args);
        }
        // else throw exception
    }

    /**
     * @param mixed $callback
     */
    public function setCallback($callback)
    {
        $this->callback = $callback;
    }

    public function getSource()
    {
        for ($i = 0; $i<100; $i++) {
            if ($this->callback) {
                $this->callback($i);
            }
        }
    }
}

CallbackSetter.php

class Provider 
{
    protected $executor;

    function __construct(CallbackExecutor $executor)
    {
        $this->executor = $executor;
    }

    public function getAll()
    {
        $app = $this;
        $this->executor->setCallback(function($i) use ($app) {
            return $app->process($i);
        });

        $this->executor->getSource();
    }
    
    protected function process($arg)
    {
        echo $i;
    }
}