faragly

class MyClass {
    private $data = [];
    private static $instance;

    public function __construct(array $config = []) {}

    public static function init(array $config = [])
    {
        if(!isset(self::$instance)) { 
            $c = __CLASS__;
            self::$instance = new $c($config); 
        } 
        return self::$instance; 
    }

    public function get()
    {
        return $this;
    }

    public function toJson()
    {
        echo json_encode($this->data);
    }
}

$myclass = new MyClass(['id' => 123]);
$myclass->get()->toJson();