code: public function handleUseRequest ($data, $transactionId, $transactionKey)
  	{
  		if (!$this->objCredits) {
  			return null;
  		}
  
  		if (isset ($_POST['transaction_id']) && isset ($_POST['transaction_secret']))
  		{
  			$valid = $this->objCredits->isRequestValid ($_POST['transaction_id'], $_POST['transaction_secret']);
  
  			if ($valid)
  			{
  				$amount = $_POST['transaction_amount'];
  				$this->objUser->useCredit ($amount, $data);
  
  				return true;
  			}
  			else
  			{
  				$this->error = 'This request was not valid or already executed. Ignore.';
  			}
  		}
  		else
  		{
  			$this->error = 'No post data received.';
  		}
  
  		return false;
  	}
docs: This function connects to the credit gateway
  and checks if a certain transaction exists.
  
  If the transaction exists, it handles it
  and returns TRUE.
  
  @param $data
  @param $transactionId
  @param $transactionKey
  @return bool|null