code: public function offsetUnset($offset)
      {
          if (property_exists($this, $offset)) {
              $this->$offset = null;
          } else {
              unset($this->$offset);
          }
      }
docs: Sets the element value at the specified offset to null.
  This method is required by the SPL interface [[\ArrayAccess]].
  It is implicitly called when you use something like `unset($model[$offset])`.
  @param mixed $offset the offset to unset element