POST using cURL and x-www-form-urlencoded in PHP returning Access Denied

I have been able to use the Advanced Rest Client Extension for chrome to send POST queries to an specific HTTPS server and I get Status Code: 200 - OK with the same body fields as the ones I used in this code, but when I run the following code I get this response: 403 - Access Denied.

<?php
$postData = array(
'type' => 'credentials',
'id' => 'exampleid',
'secret_key' => 'gsdDe32dKa'
);

// Setup cURL
$ch = curl_init('https://www.mywebsite.com/oauth/token');
curl_setopt_array($ch, array(
CURLOPT_POST => TRUE,
CURLOPT_RETURNTRANSFER => TRUE,
CURLOPT_HTTPHEADER => array('Content-Type: application/x-www-form-urlencoded'
),
CURLOPT_POSTFIELDS => json_encode($postData)
));

// Send the request
$response = curl_exec($ch);

var_dump($response);

// Check for errors
if($response === FALSE){
die(curl_error($ch));
}

// Decode the response
$responseData = json_decode($response, TRUE);

// Print the date from the response
echo $responseData['published'];
?>

I've noticed as well that when I use Advanced Rest Client Extension for chrome and if I set the Content-Type to application/json I have to enter a login and a password that I don't know what are those because even if I enter the id and secret key that I have in the code it returns 401 Unauthorized. So I'm guessing this code that I wrote is not forcing it to the content-type: application/x-www-form-urlencoded, but I'm not sure. Thank you for any help on this issue!