Use static property instead of static variable

I think better to use OOP static property for Singleton class like in other resources, isn't it?
- [domnikl/DesignPatternsPHP Singleton](https://github.com/domnikl/DesignPatternsPHP/blob/master/Creational/Singleton/Singleton.php#L13)
- [Singleton Wiki](https://uk.wikipedia.org/wiki/%D0%9E%D0%B4%D0%B8%D0%BD%D0%B0%D0%BA_(%D1%88%D0%B0%D0%B1%D0%BB%D0%BE%D0%BD_%D0%BF%D1%80%D0%BE%D0%B5%D0%BA%D1%82%D1%83%D0%B2%D0%B0%D0%BD%D0%BD%D1%8F)#.D0.A0.D0.B5.D0.B0.D0.BB.D1.96.D0.B7.D0.B0.D1.86.D1.96.D1.8F_.D0.BD.D0.B0_PHP5)