Flarum\Core\User::setPreference PHP Method

    public function setPreference($key, $value)
    {
        if (isset(static::$preferences[$key])) {
            $preferences = $this->preferences;
            if (!is_null($transformer = static::$preferences[$key]['transformer'])) {
                $preferences[$key] = call_user_func($transformer, $value);
            } else {
                $preferences[$key] = $value;
            }
            $this->preferences = $preferences;
        }
        return $this;
    }