gistfile1.php

<?php
$options = array(
    'loginId' => 'Authorize.net Login ID',
    'transactionKey' => 'Authorize.net Transaction Key',
    'mode' => 'test'
);
$gateway = Galahad_Payment::factory('AuthorizeNet', $options);

$customer = new Galahad_Payment_Customer(array(
    'firstName' => 'John',
    'lastName' => 'Smith',
    'postalCode' => '19106',
));

$card = new Galahad_Payment_Method_CreditCard('4111111111111111', 11, 2012);

$transaction = new Galahad_Payment_Transaction();
$transaction->setBillingCustomer($customer);
$transaction->setPaymentMethod($card);
$transaction->setAmount(3.50);
$transaction->setComments("I need about three-fiddy.");

$response = $gateway->process($transaction);
if ($response->isApproved()) {
    echo "Your transaction was approved!";
}
?>