Refunding a captured authorization - php

Question

am using the php api sdk found here: https://github.com/paypal/PayPal-PHP-SDK but am having trouble issuing a refund.
This is the code I am using:
// CAPTURE THE ORIGINAL AUTHORIZATION
try
{
$amount = new Amount();
$amount->setCurrency("USD");
$amount->setTotal($amount_authorized);
$capture = new Capture();
$capture->setId($authorization_id);
$capture->setAmount($amount);
$authorization = Authorization::get($transaction_id, $apiContext);
$capt = $authorization->capture($capture, $apiContext);
if ($capt->state == 'completed')
{
$capture_id = $capt->id;
}
}
catch (PayPal\Exception\PayPalConnectionException $e)
{
$response = json_decode($e->getData());
die('Error capturing funds: '.$response->message);
}
catch (Exception $e)
{
die('Error capturing funds: '.$e->getMessage());
}
// REFUND THE CAPTURE
try
{
$amount = new Amount();
$amount->setCurrency("USD");
$amount->setTotal($refund_amount);
$refund = new Refund();
$refund->setId($capture_id);
$refund->setAmount($amount);
$capture = Capture::get($capture_id, $apiContext);
$captureRefund = $capture->refund($refund, $apiContext);
if ($captureRefund->state == 'completed')
{
die('REFUNDED');
}
}
catch (PayPal\Exception\PayPalConnectionException $e)
{
$response = json_decode($e->getData());
die('There was an error refunding the funds: '.$response->message);
}
catch (Exception $e)
{
die('There was an error refunding the funds: '.$e->getMessage());
}
I get this error everytime: The requested resource ID was not found
I believe I am following the sample code on the developer site but I'll pulling my hair out. It's probably something pretty easy I hope.
Any ideas?
Thanks