LRU (Last Recently Used) cache

lru.php

<?php

/**
 * LRU is a system which cache data used last recently.
 * see more: http://www.slideshare.net/t_wada/tddbc-exercise
 */

class LRU
{
    protected $_cache = array();
    protected $_cacheSize;

    public function __construct($size) {
        $this->_cacheSize = $size;
    }

    public function set($key, $value) {
        $this->_cache[$key] = $value;
        if (count($this->_cache) > $this->_cacheSize) {
            array_shift($this->_cache);
        }
    }

    public function get($key) {
        if ( ! isset($this->_cache[$key])) {
            return null;
        }

        // Put the value gotten to last.
        $tmpValue = $this->_cache[$key];
        unset($this->_cache[$key]);
        $this->_cache[$key] = $tmpValue;

        return $this->_cache[$key];
    }
}

test.php

<?php

require('lru.php');

$cacheSize = 2;
$lru = new LRU($cacheSize);

$data = array(
    'a' => 'dataA',
    'b' => 'dataB',
    'c' => 'dataC',
);

foreach ($data as $key => $value) {
    $lru->set($key, $value);
}

var_dump($lru->get('a'));
var_dump($lru->get('b'));
$lru->set('d', 'dataD');
var_dump($lru->get('c'));
var_dump($lru->get('d'));

Good but...
If the user specifies a numeric key, it doesn't work.