Instance as a static class property

Is it possible to declare an instance of a class as a property in PHP?

Basically what I want to achieve is:

abstract class ClassA() 
{
  static $property = new ClassB();
}

Well, I know I can't do that, but is there any workaround beside always doing something like this:

if (!isset(ClassA::$property)) ClassA::$property = new ClassB();