Oauth2 access token returns "invalid client" error - api

Question

I have a problem in getting access token. After getting auth code, when I called my get_access_token it returns "invalid_client" error. I researched about it but nothing helped me. Please, look at my code and help me to solve this problem. Thank you in advance.
Here is my code:
public function get_access_token($zoho_code)
{
$headers = array(
);
$taskurl = 'https://accounts.zoho.com/oauth/v2/token';
$cdata = array(
'code' => $zoho_code,
'grant_type' => 'authorization_code',
'client_id' => $this->client_id,
'client_secret' => $this->client_secret_id,
'redirect_uri' => 'http://localhost/callback.php',
'scope' => 'ZohoMail.accounts.UPDATE,ZohoMail.accounts.READ,ZohoMail.partner.organization.READ,ZohoMail.partner.organization.UPDATE,ZohoMail.organization.accounts.CREATE,ZohoMail.organization.accounts.UPDATE,ZohoMail.organization.accounts.READ,ZohoMail.organization.domains.CREATE,ZohoMail.organization.domains.UPDATE,ZohoMail.organization.domains.DELETE,ZohoMail.organization.domains.READ',
'state' => '55555sfdfsdfgbcv',
);
$curlresult = $this->docurl($taskurl, $cdata, $headers);
return $curlresult;
}
public function docurl($taskurl, $cdata, $headers, $method = 'post',$sendjson=true) {
$ch = curl_init();
if ($method == 'get') {
if ($cdata) {
$query = '?' . http_build_query($cdata);
$taskurl .= $query;
}
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, 'GET');
} elseif ($method == 'delete') {
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, 'DELETE');
} elseif ($method == 'put') {
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, 'PUT');
} elseif ($method == 'patch') {
if($sendjson) $cdata = json_encode($cdata);
curl_setopt($ch, CURLOPT_POSTFIELDS, $cdata);
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, 'PATCH');
} else {
if($sendjson) $cdata = json_encode($cdata);
curl_setopt($ch, CURLOPT_POSTFIELDS, $cdata);
}
curl_setopt($ch, CURLOPT_URL, $taskurl);
curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, TRUE);
$res = curl_exec($ch);
$information = curl_getinfo($ch);
print_r($information);
print_r($cdata);
curl_close($ch);
$resj = json_decode($res);
return $resj;
}

Answer 1

If I read your code correctly, you are sending your client_secret as part of a POST request body encoded as JSON.
You should make a POST request with application/x-www-form-urlencoded body and you should include the Authorization header with the client_secret encoded in the Basic scheme. For more info, see the OAuth2 RFC.

Answer 2

You need to change accounts.zoho.com according to your data centre. For example accounts.zoho.in for India. You can see this in the URL of your Zoho web app. It is written in their docs link.