Symfony - 从自定义类获取根目录路径

Question

I declare a new class which not inherit from Controller. Now, I want to pull images from remote path, and store them in directory. How do I get root path?

class UserInfo
{
    public function getUserAvatar($url)
    {
        $curl = curl_init();
        curl_setopt($curl, CURLOPT_URL, $this->m_avatarOrigin); // $this->m_avatarOrigin is the remote path to pull user avatar.
        curl_setopt($curl, CURLOPT_RETURNTRANSFER, 1);
        curl_setopt($curl, CURLOPT_HEADER, true);
        curl_setopt($curl, CURLOPT_NOBODY, false);

        $response = curl_exec($curl);
        $header = '';
        $body = '';

        if (curl_getinfo($curl, CURLINFO_HTTP_CODE) == '200') {
            $headerSize = curl_getinfo($curl, CURLINFO_HEADER_SIZE);
            $header = substr($response, 0, $headerSize);
            $body = substr($response, $headerSize);
        }

    curl_close($curl);
    $fileName = ROOT_PATH."/../resources/avatar/".$this->m_name.".jpg"; // I want to get "ROOT_PATH"
    file_put_content($fileName, $body);
}

This class doesn't inherit from Controller, so I cannot use the function:

$this->get('kernel')->getRootDir();

Is there any better way to let me get root directory path?