Request UPS access token using cURL and PHP

$client_id = '######';
$client_secret = '######';

$UPS_credentials = base64_encode("$client_id:$client_secret");
$UPS_url = 'https://onlinetools.ups.com/security/v1/oauth/token?';

$curl = curl_init();
curl_setopt($curl, CURLOPT_URL, $UPS_url);
curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
curl_setopt($curl, CURLOPT_CUSTOMREQUEST, 'POST');
curl_setopt($curl, CURLOPT_POSTFIELDS, 'grant_type=client_credentials');
curl_setopt($curl, CURLOPT_HTTPHEADER, [
    "accept: application/json",
    "x-merchant-id: $client_id",
    "Authorization: Basic $UPS_credentials",
    "Content-Type: application/x-www-form-urlencoded"
]);

$error = curl_error($curl);
$response = curl_exec($curl);
curl_close($curl);

if($error){
echo $error;

}else{

$data = json_decode($response);
$UPS_get_token_status = $data -> status;

if($UPS_get_token_status !== "approved"){

echo 'Error: Token request not approved.';

}else{

// Token as been created
$UPS_access_token = $data -> access_token;
// do stuff here...

}
}