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.