Symfony\Component\Config\Definition\PrototypedArrayNode::mergeValues PHP Method

    protected function mergeValues($leftSide, $rightSide)
    {
        if (false === $rightSide) {
            // if this is still false after the last config has been merged the
            // finalization pass will take care of removing this key entirely
            return false;
        }
        if (false === $leftSide || !$this->performDeepMerging) {
            return $rightSide;
        }
        foreach ($rightSide as $k => $v) {
            // prototype, and key is irrelevant, so simply append the element
            if (null === $this->keyAttribute) {
                $leftSide[] = $v;
                continue;
            }
            // no conflict
            if (!array_key_exists($k, $leftSide)) {
                if (!$this->allowNewKeys) {
                    $ex = new InvalidConfigurationException(sprintf('You are not allowed to define new elements for path "%s". ' . 'Please define all elements for this path in one config file.', $this->getPath()));
                    $ex->setPath($this->getPath());
                    throw $ex;
                }
                $leftSide[$k] = $v;
                continue;
            }
            $prototype = $this->getPrototypeForChild($k);
            $leftSide[$k] = $prototype->merge($leftSide[$k], $v);
        }
        return $leftSide;
    }