<?php

class Foo {

  private $values;

  public function __construct() {

    $this->values = array();

  }

  public function addElement($key, $value) {

    $this->values[$key] = $value;

  }

  public function getValues() {

    return $values;

  }

}

$foo = new Foo();
$foo->addElement("hello", "John Doe");
$foo->getValues()["goodbye"] = "Jane Doe";

var_dump($foo);