I just ran into this exact same problem today. My script was working perfectly, then it just stopped. It turned out I had to decode the response and then extract the access token from the decoded response. Otherwise, $accessToken was an empty string, thus invalid. This is how I rewrote my code.
$response = json_decode($response);
$accessToken = $response->access_token;
This implies something changed with the API response for access tokens. I’m not sure what, but this fixed it immediately.