构造函数[duplicate]中不存在PHP类

致命错误: 对非对象调用成员函数query()

SPConfig SPClasses

$sp = new SPClasses();

class SPClasses
{
    public $db, $config, $page, $users;

    function __construct()
    {       
        // The SPDatabase class [...]
        $this->db = new SPDatabase();

        // The SPConfig class [...]
        $this->config = new SPConfig();

        // [...]

        // The SPTemplate class [...]
        $this->page = new SPTemplate();

        // The SPUsers class [...]
        $this->users = new SPUsers();
    }
}

class SPConfig
{
    function __construct()
    {
        global $sp;
        $configquery = $sp->db->query("SELECT * FROM config", 'Retrieving the configuration');
        while($row = mysql_fetch_array($configquery))
        {
            try
            {
                $this->{$row['name']} = $row['value'];
            }
            catch (Exception $e)
            {
                $debug_log[] = array(
                    'type' => 'error',
                    'success' => false,
                    'text' => 'The config setting <i>' . $row['config_value'] . '</i> has an invalid name.'
                );
            }
        }
    }


    public function update()
    {
        $this->__construct();
    }
}

有人知道为什么我不能进入吗 $sp->db 建造师?

谢谢。