Access and refresh token (JWT) php

Question

I'm trying to use JWT tokens in my php project with php-jwt library.

At now I can log in with smartphone app. After success registration app gets jwt access_token by JWT::encode($token, $key);. Client can send this token to the server, which checks it and e.g changes database data.

I have several questions at once.

1. How can I create correct values for JWT:

$key = "your_secret_key";
$iss = "http://any-site.org";
$aud = "http://any-site.com";
$iat = 1356999524;
$nbf = 1357000000;

How can I see:

 $key - any combination of characters like salt.
 $iss - site address, which sends tokens (my server location)
 $aud - same address
 $iat - creation time (do I need to change it every time when access token is created?) 
 $nbf - token life time (what time should I use?)

2. How can I create refresh token?

access_token — the token we send with headers in every request.

refresh_token — the token we send with headers when the old token lifetime will expire.

3. Where do I should to store both tokens?
   I guess, that access_token can be stored in smartphone's memory and sent it every time, when the server needs to check access.

4. How can I save the user's authorization after each app launch?

Thank you for your advice!