This method returns true only if the parameter's default value is set to a named constant.  e.g.
class Foo {
   const OPTION_MULTIGET = 1;
   //Method 1
   public function setGetOptions($opts=0);
   //Method 2
   public function setMultiGetOptions($opts=Foo::OPTION_MULTIGET);
}
Method 1's first ReflectionParameter will return isDefaultValueConstant() = false (0 is a "constant"/static value but not a named constant) and Method 2's will return isDefaultValueConsntant() = true.