ZOHO Book Access+Refresh Token

Hi,

I have written PHP code to generate oAuth token for zohobook (india region) for offline access and this process working perfectly fine. Whenever I try to create refresh token and access token, it is always giving me access token only, and there is no refresh token given, also next time when I try to generate access token again, it is saying invalid code, meaning the auth token generated with oAuth is no longer valid.

Here is the URL for oAuth

https://accounts.zoho.in/oauth/v2/auth?client_id=1000.***&response_type=code&scope=ZohoBooks.fullaccess.all&redirect_uri=http://jayshree.local/site/oauthclient&access_type=offline

And here is the code for refresh and access token

$curl = curl_init();
$client_id=1000.*****;
$client_secret=6****;
$redirect_uri=http://jayshree.local/site/oauthclient;
$auth_token = TOKEN_GENERATED_FROM_OAUTH_URL;

        curl_setopt_array($curl, 
            array(
                CURLOPT_URL => $url,
                CURLOPT_RETURNTRANSFER => true,
                CURLOPT_ENCODING => '',
                CURLOPT_MAXREDIRS => 10,
                CURLOPT_TIMEOUT => 0,
                CURLOPT_FOLLOWLOCATION => true,
                CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
                CURLOPT_CUSTOMREQUEST => 'POST',
                CURLOPT_POSTFIELDS => 'client_id='.$client_id.
                                      '&grant_type=authorization_code'.
                                      '&client_secret='.$client_secret.
                                      '&redirect_uri='.$redirect_uri.
                                      '&code='.$auth_token,
                CURLOPT_HTTPHEADER => array(
                    'Content-Type: application/x-www-form-urlencoded'
                ),
            )
        );

        $response = curl_exec($curl);

        curl_close($curl);

This returns to me "invalid_code" error always. Please suggest what is wrong and what should be corrected