What is ReflectionClass::IS_IMPLICIT_ABSTRACT used for?

I'm trying to understand the PHP ReflectionClass but have some trouble. In the documentation 3 modifiers for classes are listed: implicit abstract, explicit abstract and final with constant values 16, 32 and 64 respectively. What I wonder is how the implicit and explicit abstract modifiers work? When is a PHP class implicit abstract?

I tried to figure it out by trial and error with a few simple classes and interfaces and using the ReflectionClass::getModifiers() method to see what happened, however I do not understand the result of the getModifiers method. Using the following interface the result from getModifiers is int(144):

interface Animal {
  public function isYellow();
}

$a = new ReflectionClass("Animal");
var_dump($a->getModifiers());

With the modifier constants being 16, 32 and 64 respectively I don't get how the method can return 144 at all? Have I misunderstood something about how a bitfield works or why am I getting this result?