PHP - Send post data to URL and get the response

Question

I'm having a hard time trying to send POST data to the twitch api (https://api.twitch.com/kraken/oauth2/token) and get back the json code... This is my current code:

$this->config->load("config", true);
$url = "https://api.twitch.tv/kraken/oauth2/token";
$postvars = "client_id=" . $this->config->item('twitch_clientid', 'config') . "&client_secret=" . $this->config->item('twitch_secret', 'config') . "&grant_type=user_read&redirect_uri=" . $this->config->item('twitch_redirect', 'config') . "&code=" . $key;

$temp = curl_init($url);
curl_setopt($temp, CURLOPT_POST, 1);
curl_setopt($temp, CURLOPT_POSTFIELDS, $postvars);
curl_setopt($temp, CURLOPT_FOLLOWLOCATION, 1);
curl_setopt($temp, CURLOPT_HEADER, 0);
curl_setopt($temp, CURLOPT_RETURNTRANSFER, 1);
$result = json_decode(@file_get_contents(curl_exec($temp)), true);

But... obviously that is not the way I should be doing it. First time working with curl.
Error:

A PHP Error was encountered 
Severity: Warning
Message: file_get_contents(): Filename cannot be empty
Filename: controllers/user.php
Line Number: 37

Result expected:

{
  "access_token": "[user access token]",
  "scope":[array of requested scopes]
}

Answer 1

The error is in this line:

$result = json_decode(@file_get_contents(curl_exec($temp)), true);

curl_exec returns false or data when returntransfer is on (https://www.php.net/curl_exec) therefore the code should look more like

if (! $data = curl_exec($ch)) { 
    //ERROR 
} else {
    $result = json_decode($data, true);
}
curl_close($ch); 

You can always set option VERBOSE to true, to have a better view of what is send to the server.