i18n::getUserLangs PHP Method

    public function getUserLangs()
    {
        $userLangs = array();
        // Highest priority: forced language
        if ($this->forcedLang != NULL) {
            $userLangs[] = $this->forcedLang;
        }
        // 2nd highest priority: GET parameter 'lang'
        if (isset($_GET['lang']) && is_string($_GET['lang'])) {
            $userLangs[] = $_GET['lang'];
        }
        // 3rd highest priority: SESSION parameter 'lang'
        if (isset($_SESSION['lang']) && is_string($_SESSION['lang'])) {
            $userLangs[] = $_SESSION['lang'];
        }
        // 4th highest priority: HTTP_ACCEPT_LANGUAGE
        if (isset($_SERVER['HTTP_ACCEPT_LANGUAGE'])) {
            foreach (explode(',', $_SERVER['HTTP_ACCEPT_LANGUAGE']) as $part) {
                $userLangs[] = strtolower(substr($part, 0, 2));
            }
        }
        // Lowest priority: fallback
        $userLangs[] = $this->fallbackLang;
        // remove duplicate elements
        $userLangs = array_unique($userLangs);
        // remove illegal userLangs
        $userLangs2 = array();
        foreach ($userLangs as $key => $value) {
            // only allow a-z, A-Z and 0-9 and _ and -
            if (preg_match('/^[a-zA-Z0-9_-]*$/', $value) === 1) {
                $userLangs2[$key] = $value;
            }
        }
        return $userLangs2;
    }