php单例模式

class SingleDesign
{
    private static $instance = null;

    private function __construct(){}
    private function __clone(){}

    public static function getInstance()
    {
        if (self::$instance === null) {
            self::$instance = new self();
        }
        return self::$instance;
    }

}

function user_info ($uid)
{
    static $s_user = array();
    if (isset($s_user[$uid])) {
        $s_user[$uid] = $db->where("id=" . $uid)->query();
    }
    return  $s_user[$uid];
}