public function addTag($tagName, $tagValue)
    {
        if (!isset($this->tags)) {
            $this->tags = [];
        }

        // The same tag cannot be added 2 times
        if (array_key_exists($tagName, $this->tags)) {
            throw new PushwooshException(
                'The tag \'' . $tagName .
                '\' has already been added, use the \'setTag\' method if you want to overwrite its value !'
            );
        }

        $this->tags[$tagName] = $tagValue;
    }