Settings: Key => value as Singleton

Settings.php

<?php

class Settings {
    var $values;
    final public static function getInstance() {
        static $aoInstance = null;
        if (is_null($aoInstance)) {
            $aoInstance = new Settings();
        }
        return $aoInstance;
    }
    public function __construct() {
        $this->values = array( );
    }
    function setValue($key, $val ) {
        $this->values [$key] = $val;
        return $this;
    }
    function getValue($key, $default = null) {
        return (isset($this->values[$key])) ? $this->values[$key] : $default;
    }
}