code: protected static function getStaticVar($staticVarName)/*# : array */
      {
          $class = get_called_class();
          $parent = get_parent_class($class);
  
          // get current class' static variable
          $res = $class::${$staticVarName};
  
          // merge with ancestor class' same static variable
          if ($parent && property_exists($parent, $staticVarName)) {
              $res = $parent::getStaticVar($staticVarName);
              if ($class::${$staticVarName} != $parent::${$staticVarName}) {
                  $res = array_replace_recursive($res, $class::${$staticVarName});
              }
          }
  
          return $res;
      }
docs: Get MERGED/REPLACED static variable for CURRENT class
  
  @param  string $staticVarName static variable name
  @return array
  @access protected
  @static