self::_default = new self() is can't run

[LSZ]

class AutoRegister {
    public static _default;
    private path;
    public static function init(string path) ->void {
        let this->path = path;
        if self::_default === NULL {
            let self::_default = new self();
        }
    }

I can`t run these code. what wrong whith these code?

 if self::_default === NULL {
            let self::_default = new self();
        }

[LSZ]

sorry,I had know reason, because I can't use not static params in static function.