Refresh Token for Quickbooks expiring after 1 day - php

Question

I've built an app that connects to Quickbooks using OAuth 2 with the official Quickbooks PHP SDK.
I get two tokens:
Access token (Expiring after 60 minutes)
Refresh token (Expiring after 100 days)
The Refresh token is used to get a new Access token.
However, by requesting a new access token I also receive a new refresh token that I save in the database.
The previous refresh token will be forced to expire. But the new refresh token is invalid already.
I have built a CRON job that request a new refresh token every 15 minutes. I do get a new refresh token but this one is invalid (Error : "invalid_grant"). (See the code below)
What am I doing wrong here ?
$dataService->setMinorVersion("9");
$OAuth2LoginHelper = $dataService->getOAuth2LoginHelper();
$accessToken = $OAuth2LoginHelper->refreshToken();
$dataService->updateOAuth2Token($accessToken);
$token->previous_token = $token->last_token;
$token->last_token = $accessToken->getRefreshToken();
if($token->last_token != $token->previous_token){
if($token->save()){
mail('mymail', 'New token !', "Token updated!!!!");
}
}