Piwik\Plugin\Manager::isPluginInstalled PHP Method

    public function isPluginInstalled($pluginName)
    {
        $pluginsInstalled = $this->getInstalledPluginsName();
        return in_array($pluginName, $pluginsInstalled);
    }

 private function isPluginInstalled($pluginName)
 {
     if (in_array($pluginName, $this->activatedPluginNames)) {
         return true;
     }
     return $this->pluginManager->isPluginInstalled($pluginName);
 }