Contao\CoreBundle\EventListener\StoreRefererListener::prepareBackendReferer PHP Method

    private function prepareBackendReferer($refererId, array $referers = null)
    {
        if (!is_array($referers)) {
            $referers = [];
        }
        if (!isset($referers[$refererId]) || !is_array($referers[$refererId])) {
            $referers[$refererId] = ['last' => ''];
        }
        // Make sure we never have more than 25 different referer URLs
        while (count($referers) >= 25) {
            array_shift($referers);
        }
        return $referers;
    }