( excerpt from http://php.net/manual/en/reflectionclass.getdefaultproperties.php )
public function getDefaultProperties(): darray<string, mixed>;
Gets default properties from a class (including inherited properties).
This method only works for static properties when the default value is known statically. If it is not known statically you will get null instead of the correct value. Do not rely on this API for default values of static properties.
mixed- An array of default properties, with the key being the name of the property and the value being the default value of the property or NULL if the property doesn't have a default value. The function does not distinguish between static and non static properties and does not take visibility modifiers into account.