Pimcore\API\Plugin\Broker::registerPlugin PHP Method

    public function registerPlugin(AbstractPlugin $plugin, $stackIndex = null)
    {
        if (false !== array_search($plugin, $this->_plugins, true)) {
            throw new \Exception('Plugin already registered');
        }
        //installed?
        if (!$plugin::isInstalled()) {
            if (is_object($plugin)) {
                $className = get_class($plugin);
                Logger::debug("Not registering plugin [ " . $className . " ] because it is not installed");
            } else {
                Logger::debug("Not registering plugin, it is not an object");
            }
            return $this;
        }
        $stackIndex = (int) $stackIndex;
        if ($stackIndex) {
            if (isset($this->_plugins[$stackIndex])) {
                throw new \Exception('Plugin with stackIndex "' . $stackIndex . '" already registered');
            }
            $this->_plugins[$stackIndex] = $plugin;
        } else {
            $stackIndex = count($this->_plugins);
            while (isset($this->_plugins[$stackIndex])) {
                ++$stackIndex;
            }
            $this->_plugins[$stackIndex] = $plugin;
        }
        ksort($this->_plugins);
        $plugin->init();
        return $this;
    }