The CodeIgniter Helper handles many credit card checks which can be used to manipulate credit card numbers and related information

credit_card_helper.php

<?php  if (!defined('BASEPATH')) exit('No direct script access allowed');


/**
 * Credit Card Functions
 *
 * This helper module contains functions which can be used to manipulate credit 
 * card numbers and related information.
 *
 * @package     CodeIgniter
 * @subpackage  Helpers
 * @category    Helpers
 * @author      Jim O'Halloran <jim@tuxit.com.au>
 * @author      Jason Gegere <jason@hgmail.com>
 */


/**
 * Truncates a card number retaining only the first 4 and the last 4 digits.  It then returns the truncated form.
 *
 * @return array of years.
 */
if( ! function_exists( 'getYears' ) ){
    function getYears() {
        $years = array();
        $curYear = date("Y");
        $limit = 10;
        for ($x = $curYear; $x < $curYear + $limit; $x++) {
            $years[$x] = $x;
        }
        return $years;
    }
}

/**
 * Truncates a card number retaining only the first 4 and the last 4 digits.  It then returns the truncated form.
 *
 * @param string The card number to truncate.
 * @return string The truncated card number.
 */
if( ! function_exists( 'truncate_card' ) ){
    function truncate_card($card_num) {
        $padsize = (strlen($card_num) < 7 ? 0 : strlen($card_num) - 7);
        return substr($card_num, 0, 4) . str_repeat('X', $padsize). substr($card_num, -3);
    }
}


/**
 * Validates a card expiry date.  Finds the midnight on first day of the following 
 * month and ensures that is greater than the current time (cards expire at the 
 * end of the printed month).  Assumes basic sanity checks have already been performed 
 * on month/year (i.e. length, numeric, etc).
 *
 * @param integer The expiry month shown on the card.
 * @param integer The expiry year printed on the card.
 * @return boolean Returns true if the card is still valid, false if it has expired.
 */
if( ! function_exists( 'card_expiry_valid' ) ){
    function card_expiry_valid($month, $year) {
        $expiry_date = mktime(0, 0, 0, ($month + 1), 1, $year);
        return ($expiry_date > time());
    }
}


/**
 * Strips all non-numerics from the card number.
 *
 * @param string The card number to clean up.
 * @return string The stripped down card number.
 */
if( ! function_exists( 'card_number_clean' ) ){
    function card_number_clean($number) {
        return preg_replace("/[^0-9]/", "", $number); 
    }
}

/**
 * Uses the Luhn algorithm (aka Mod10) <http://en.wikipedia.org/wiki/Luhn_algorithm> 
 * to perform basic validation of a credit card number.
 *
 * @param string The card number to validate.
 * @return boolean True if valid according to the Luhn algorith, false otherwise.
 */
if( ! function_exists( 'card_number_valid' ) ){
    function card_number_valid ($card_number) {
        $card_number = strrev(card_number_clean($card_number));
        $sum = 0;
        
        for ($i = 0; $i < strlen($card_number); $i++) {
          $digit = substr($card_number, $i, 1);
        
            // Double every second digit
            if ($i % 2 == 1) {
              $digit *= 2;
            }
            
            // Add digits of 2-digit numbers together
            if ($digit > 9)    {
              $digit = ($digit % 10) + floor($digit / 10);
            }
            
            $sum += $digit;
        }
        
        // If the total has no remainder it's OK
        return ($sum % 10 == 0);
    }

}

/* 
End of file: credit_card_helper.php
Location: application/helpers/credit_card_helper.php 
*/

Hello @gstjohn I wanted to extend a blog post you wrote about the credit card helper @jimohalloran created. I have taken it and extended it. I didn't want to create a complete repo for one page and thought a gist would be a perfect place to start.