Google OAuth2 error - Required parameter is missing: grant_type on refresh

Question

I have built a prototype calendar synching system using the Google calendar API and it works well, except refreshing access tokens. These are the steps I have gone through:

1) Authorised my API and received an authorisation code.

2) Exchanged the authorisation code for Access Token and a RefreshToken.

3) Used the Calendar API until the Access Token expires.

At this point I try to use the Refresh Token to gain another Access Token, so my users don't have to keep granting access because the diary sync happens when they are offline.

Here's the PHP code, I'm using curl requests throughout the system.

$requestURL = "https://accounts.google.com/o/oauth2/token";
$postData = array("grant_type" => "refresh_token", 
                  "client_id" => $clientID,    
                  "client_secret" => $clientSecret, 
                  "refresh_token" => $refreshToken);

$headers[0] = 'Content-Type: application/json';

$ch = curl_init();
curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
curl_setopt($ch, CURLOPT_URL, $requestURL);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($postData));

$response = curl_exec($ch);
$responseArray = json_decode($response, TRUE);

The response i'm getting is:

[error] => invalid_request
[error_description] => Required parameter is missing: grant_type

No curl errors are reported.

I've tried header content-type: application/x-www-form-urlencoded, and many other things, with the same result.

I suspect it's something obvious in my curl settings or headers as every parameter mentioned in the Google documentation for this request is set. However, I'm going around in circles so would appreciate any help, including pointing out any obvious errors I've overlooked.

Answer 1

your request should not post JSON data but rather query form encoded data, as in:

$requestURL = "https://accounts.google.com/o/oauth2/token";
$postData = "grant_type=refresh_token&client_id=$clientID&client_secret=$clientSecret&refresh_token=$refreshToken";

$headers[0] = 'Content-Type: application/x-www-form-urlencoded';

$ch = curl_init();
curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
curl_setopt($ch, CURLOPT_URL, $requestURL);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, $postData);

$response = curl_exec($ch);
$responseArray = json_decode($response, TRUE);