Deprecate implicit dynamic properties

Writing to a proprety that hasn't been declared is deprecated,
unless the class uses the #[AllowDynamicProperties] attribute or
defines __get()/__set().

RFC: https://wiki.php.net/rfc/deprecate_dynamic_properties

ext/spl/tests/arrayObject___construct_basic2.phpt

@@ -54,6 +54,8 @@ function testAccess($c, $ao) {
 NULL
 string(12) "C::prop.orig"
   - Write:
+
+Deprecated: Creation of dynamic property ArrayObject::$prop is deprecated in %s on line %d
 string(8) "changed1"
 string(8) "changed2"
   - Isset: