I have a class B that extends class A

The constructor for Class never gets called.

class  A  {
       A() {
           print("Hi from Class A");
       }

       Nothing() {
           print("Class A - Nothing");
       }
}

class B  extends A {
       B()  {
           print("Hi from Class B");
       }
}


$ClassB = new B;
$ClassB->Nothing();

When I create a class B the B constructor gets called but NOT the class A constructor.  How do I get the A constructor to be called as well?