<form>
            contactId: <input type="text" name="contactId" value="<?php 
if (isset($_REQUEST['contactId'])) {
    echo htmlspecialchars($_REQUEST['contactId']);
}
?>
"><br/>
            last4: <input type="text" name="last4" value="<?php 
if (isset($_REQUEST['last4'])) {
    echo htmlspecialchars($_REQUEST['last4']);
}
?>
"><br/>
    <input type="submit">
<input type="hidden" name="go">
</form>
<?php 
include '../infusionsoft.php';
include 'testUtils.php';
if (isset($_REQUEST['go'])) {
    $out = Infusionsoft_InvoiceService::locateExistingCard($_REQUEST['contactId'], $_REQUEST['last4']);
    var_dump($out);
}