\Firebase\JWT\JWT::decode() in php is confusing me

I came across this bit of code:

class JWT {
    private $secret = "secret-string";
    private $algos = ['HS256'];
    private $domain;

    function __construct() {
        $this->domain = $_SERVER['HTTP_HOST'];
    }

    function get() {
        if ( isset($_COOKIE['jwt']) ) {
            $ticket = $_COOKIE['jwt'];
            try {
                $data = \Firebase\JWT\JWT::decode($ticket, $this->secret, $this->algos);
                return $data->data;
            } catch (Exception $e) {
                return null;
            }
        } else {
            return null;
        }
    }

    function set($data) {
        $time = time();
        $expire = $time+60*60*24;
        $data = [
            "iat" => $time,
            "nbf" => $time,
            "exp" => $expire,
            "data" => $data,
        ];
        $jwt = \Firebase\JWT\JWT::encode($data, $this->secret, $this->algos[0]);
        setcookie('jwt', $jwt, $expire, '/', $domain, true, true);
    }
}

I am not familiar with php, But because of certain conditions, I have to slam my head against it currently. After slamming my head repetitively against this, I came to the conclusion that I am having a problem in the following bit of code:

$jwt = \Firebase\JWT\JWT::encode($data, $this->secret, $this->algos[0]);
setcookie('jwt', $jwt, $expire, '/', $domain, true, true);

I am not able to understand the format the JWT tokens will be encoded in. I have tried some trial and error but I failed and I feel like I am missing somewhere. My last resort was to move to stackoverflow to ask the wonderful programmers here about this.

For context: I would have posted this on security overflow, But I posted it here as the last time I did that my question got migrated.

Hopefully I can be shown the right direction!