Uber API invalid grant error? - php

Question

I've been struggling to understand and find my way through the Uber API and their useless documentation.
I managed to put together this code in PHP which uses CURL to get an authorization_code code so I can further use that in other parts of my PHP page. For example, request a ride etc...
However, when I run my code, I get this error:
{"error": "invalid_grant"}
I searched high and low on Google and STO to see if i could find a solid answer to this but unfortunately nothing seems to give an straight forward answer and I'm stuck at this stage.
This is my code:
<?php
$fields_string = '';
$fields = array(
'client_secret' => '57KC5WGG3Ia455lc-i53e345345znXBB_0',
'client_id' => 'nnV4-_lsG9xl-SbAIQjIhwer234534534',
'grant_type' => 'authorization_code',
'redirect_uri' => "http://localhost:3000/auth/uber/callback",
'code' => $_GET['code']
);
foreach($fields as $key=>$value) { $fields_string .= $key.'='.$value.'&'; }
rtrim($fields_string, '&');
$ch = curl_init();
curl_setopt($ch,CURLOPT_URL, "https://login.uber.com/oauth/v2/token");
curl_setopt($ch,CURLOPT_POST, count($fields));
curl_setopt($ch,CURLOPT_POSTFIELDS, $fields_string);
$result = curl_exec($ch);
$responseData = json_decode($result , TRUE);
$access_token = $responseData["access_token"];
echo "Access Token :" . $access_token ;
curl_close($ch);
?>
it might be worth mentioning that, the above code is all that I have in my php page. nothing else.
Questions:
Is there something that i am missing?
Do I need to do anything in my Uber developer dashboard?
Do I need to do anything else in my php page?
Any help or pointers that is laymen terms and make sense would be appreciated.

Answer 1

Possible solutions taken from: https://developer.uber.com/docs/riders/references/api/v2/token-post
You are using an invalid refresh_token. You can generate multiple access tokens, but you can only use the latest generated refresh_token.
You supplied an invalid code when exchanging an authorization code for an access_token.