Simulate Anidated Contexts

<?php

error_reporting(E_ALL | E_STRICT);

class Context implements ArrayAccess {
    /**
     * @var Context
     */
    protected $parent;

    /**
     * @var ArrayObject
     */
    protected $current;

    public function __construct($items = array(), Context $parent = NULL) {
        $this->current = new ArrayObject((array)$items);
        $this->parent = $parent;
    }

    protected function &getRef($index) {
        $ref = &$this->current[$index];
        if (!isset($ref) && ($this->parent !== NULL)) {
            $ref = &$this->parent->getRef($index);
        }
        return $ref;
    }

    public function offsetExists($index) {
        $ref = &$this->getRef($index);
        return isset($ref);
    }

    public function &offsetGet($index) {
        return $this->getRef($index);
    }

    public function offsetSet($index, $newval) {
        $ref = &$this->getRef($index);
        $ref = $newval;
    }

    public function offsetUnset($index) {
        $ref = &$this->getRef($index);
        unset($ref);
    }

    public function enter($items = array()) {
        return new static($items, $this);
    }

    public function leave() {
        return $this->parent;
    }
}

$context = new Context(array('a' => -10, 'b' => -20, 'c' => -30));
$context = $context->enter(array('a' => 10, 'b' => 20));
printf("%d\n", ++$context['a']);
printf("%d\n", ++$context['c']);

$context = $context->leave();
printf("%d\n", ++$context['a']);
printf("%d\n", ++$context['c']);

$context['c'] = 6;

printf("%d\n", $context['c']);