ArrayChaser.php

<?php

class ArrayChaser implements ArrayAccess {
    private $array_ref;
    private $log = array();

    public function __construct(array &$ref)
    {
        $this->array = &$ref;
    }

    public function offsetExists($offset)
    {
        $this->log[] = array('ISSET', $offset);
        return isset($this->array[$offset]);
    }

    public function offsetGet($offset)
    {
        $this->log[] = array('GET', $offset);
        return $this->array[$offset];
    }

    public function offsetSet($offset, $value)
    {
        $this->log[] = array('SET', $offset);
        if (!isset($offset)) {
            $this->array[] = $value;
        } else {
            $this->array[$offset] = $value;
        }
    }

    public function offsetUnset($offset)
    {
        $this->log[] = array('UNSET', $offset);
        unset($this->array[$offset]);
    }
}

$array = array();
$ref = new ArrayChaser($array);
$ref['hoge'] = 'foo';
$ref['fuga'] = 'foo';