Joomla! RSForm Custom Validation check if giftcode exists and not used yet

customvalidation.php

<?php

defined('_JEXEC') or die('Restricted access');

require_once dirname(__FILE__) . '/validation.php';

class RSFormProCustomValidations extends RSFormProValidations
{

	public static function giftcode($value, $extra = null, $data = null)
	{
		$db = JFactory::getDBO();
		$db->setQuery("SELECT `giftcode` FROM `#__rsform_custom_giftcode`");
		$giftcodes = $db->loadColumn();

		if (self::checkstring($value, $giftcodes))
		{
			// Return false if the code does not exist
			return false;
		}

		$db->setQuery("SELECT `used` FROM `#__rsform_custom_giftcode` WHERE `giftcode` ='" . $value . "'");
		$used = $db->loadResult();

		if ($used == 1)
		{
			// Return false if the code is already used
			return false;
		}

		// Return true... the code exists and has not been used
		return true;
	}

	public static function checkstring($string, $giftcodes)
	{
		foreach ($giftcodes as $giftcode)
		{
			if (stristr($string, $giftcode))
			{
				return false;
			}
		}

		return true;
	}

}