Get PHP variables in validation

I am trying to integrate NGPVAN's contribution services with Gravity Forms. Their API takes a PHP array that looks something like this:

$d = new NgpDonation('your-credentials', true, array(
    'FirstName' => 'John',
    'LastName' => 'Doe',
    'Address1' => '100 Elm Street',
    'Zip' => '12345',
    'Cycle' => '2012',
    'Amount' => '50.00',
    'CreditCardNumber' => '4111111111111111',
    'ExpYear' => '13',
    'ExpMonth' => '05'
));

with many other possible fields. My code for validation is:

add_action("gform_validation", "process_credit_card_data");

/*
* Courtsey of
* http://www.newmediacampaigns.com/blog/accept-campaign-donations-with-the-ngp-contribution-api
* http://www.gravityhelp.com/documentation/page/Gform_validation
*/

function process_credit_card_data($validation_result) {
    // print_r($form); // for debugging
    // print_r($validation_result);
    // print_r($_POST);
    if ($validation_result['is_valid']) {
                    // cycle information
            $cycle = '2012';
        /* ******************** General Web Contribute Form ******************** */
        if (($validation_result['form']['id'] == 4)) {
            $source = 'ccOnline';
      // attempt payment -- being ready to reject if needed.
            // name information
            $firstName = rgpost("input_{$field['16']}");
            $lastName = rgpost("input_{$field['17']}");
            $email = rgpost("input_{$field['3']}");
            $phone = rgpost("input_{$field['4']}");

            // address information
                $address1 = rgpost("input_{$field['18']}");
                $address2 = rgpost("input_{$field['19']}");
                $city = rgpost("input_{$field['20']}");
                $state = rgpost("input_{$field['21']}");
                $zip = rgpost("input_{$field['22']}");

            // amount information
              $amountradio = rgpost("input_{$field['6']}");
              $amount = '5';

		    // credit card information
            $ccnum = rgpost("input_{$field['8']}");
            $exmonth = rgpost("input_{$field['9']}");
            $exyear = rgpost("input_{$field['10']}");

            // recurring information
         //   $recurring = rgpost("input_11');
          //  $recurringfreq = rgpost("input_12');
         //   $recurringtotal = rgpost("input_13');

            // employment information
            $employer = rgpost("input_{$field['14']}");
            $occupation = rgpost("input_{$field['15']}");

            $result = do_transaction($lastName, $firstName, $email, $phone, $address1, $address2, $city, $state, $zip, $amount, $ccnum, $expMonth, $expYear, $employer, $occupation, $cycle, $source);
            if ($result !== true) {
                $validation_result['is_valid'] = false;
                foreach($validation_result["form"]["fields"] as &$field){
                    if ($field["id"] == "8"){
                        $field["failed_validation"] = true;
                        $field["validation_message"] = $result;
                        break;
                    }
                }
                } else {
                // I like to blank credit card data so it's not stored locally.
                $_POST['input_8'] = '*****';
            }
            return $validation_result;
            /* ******************** End General Web Contribute Form ******************** */
            } else {
            return $validation_result;
        }
        } else {
        return $validation_result;
    }
}

 // This is the function that is called above
function do_transaction($lastName, $firstName, $email, $phone, $address1, $address2, $city, $state, $zip, $amount, $ccnum, $exmonth, $exyear, $employer, $occupation, $cycle, $source) {
    require 'NgpDonation.php';

    // Your string goes below. Will look something like the following:
    $apiString = "*******REDACTED FOR CONFIDENTIALITY********";
    $d = new NgpDonation($apiString, true, array(
    'FirstName' => $firstName,
    'LastName' => $lastName,
    'Email' => $email,
    'HomePhone' => $phone,
    'Address1' => $address1,
    'Address2' => $address2,
    'City' => $city,
    'State' => $state,
    'Zip' => $zip,
    'Cycle' => $cycle,
    'Amount' => $amount,
    'CreditCardNumber' => $ccnum,
    'ExpYear' => $exyear,
    'ExpMonth' => $exmonth,
    'Employer' => $employer,
    'Occupation' => $occupation,
    'Source' => $source
    ));
    $result = $d->save();
    if ($result) {
        return true;
        } else {
        if ( $d->hasErrors() ) {
            $errors = $d->getErrors();
            //print_r($errors);
        }
        if ( $d->hasFault() ) {
            $fault = $d->getFault();
            //print_r($fault);
        }
        $result = $d->getResult();
        $errorString = 'testingerror';
        return $errorString;
    }
}

Every time I send the form I get the validation error. Am I pulling the variables incorrectly? Any help would be greatly appreciated.