<?php
class Values {
    private $val;
    function setVal($value){ $this->val = $value; }
    function getVal(){ return $this->val; }
}
class Modify {
    private $instance;
    function __construct($instance) {
        $this->instance = $instance;
    }
    function change($val){
        $this->instance->setVal($val);
    }
}

$foo = new Values();
$foo->setVal("cats");
$bar = new Modify($foo);
$bar->change("dogs");