What do we actually mean when we say "validate a credit card number"? Quite simply it means that we run a credit card number through a special algorithm known as the Mod 10 algorithm.

This algorithm processes some simple numerical data validation routines against the number and the result of this algorithm can be used to determine whether or not a credit card number is valid. There are several different types of credit cards that one can use to make a purchase, however they can all be validated using the Mod 10 algorithm.

As well as passing the Mod 10 algorithm, a credit card number must also pass several different formatting rules. A list of these rules for each of the six most popular credit cards is shown below: Mastercard: Must have a prefix of 51 to 55, and must be 16 digits in length. Visa: Must have a prefix of 4, and must be either 13 or 16 digits in length. American Express: Must have a prefix of 34 or 37, and must be 15 digits in length. Diners Club: Must have a prefix of 300 to 305, 36, or 38, and must be 14 digits in length. Discover: Must have a prefix of 6011, and must be 16 digits in length. JCB: Must have a prefix of 3, 1800, or 2131, and must be either 15 or 16 digits in length. As mentioned earlier, in this article we will be creating a PHP class that will hold the details of a credit card number and expose a function indicating whether or not the number of that credit card is valid (i.e. whether it passed the Mod 10 algorithm or not). Before we create that class however, let's look at how the Mod 10 algorithm works.

The Mod 10 algorithm There are three steps that the Mod 10 algorithm takes to determine whether or not a credit card number is valid. We will use the valid credit card number 378282246310005 to demonstrate these steps:

Step One The number is reversed and the value of every second digit is doubled, starting with the digit in second place:

378282246310005
... becomes ...
500013642282873

and the value of every second digit is doubled:

5 0 0 0 1 3 6 4 2 2 8 2 8 7 3 x2 x2 x2 x2 x2 x2 x2
-------------------------------------------
0 0 6 8 4 4 14

Step Two The values of the numbers that resulted from multiplying every second digit by two are added together (i.e. in our example above, multiplying the 7 by two resulted in 14, which is 1 + 4 = 5). The result of these additions is added to the value of every digit that was not multiplied (i.e. the first digit, the third, the fifth, etc):

5 + (0) + 0 + (0) + 1 + (6) + 6 + (8) + 2 + (4) + 8 + (4) + 8 + (1 + 4) + 3

= 60

Step Three When a modulus operation is applied to the result of step two, the remainder must equal 0 in order for the number to pass the Mod 10 algorithm. The modulus operator simply returns the remainder of a division, for example:

10 MOD 5 = 0 (5 goes into 10 two times and has a remainder of 0)
20 MOD 6 = 2 (6 goes into 20 three times and has a remainder of 2)
43 MOD 4 = 3 (4 goes into 43 ten times and has a remainder of 3)

So for our test credit card number 378282246310005, we apply a modulus of 10 to the result from step two, like this:

60 MOD 10 = 0

The modulus operation returns 0, indicating that the credit card number is valid.

Now that we understand the Mod 10 algorithm, it's really quite easy to create our own version to validate credit card numbers with PHP. Let's create our credit card class now.

Creating the CCreditCard Class
Let's now create a PHP class that we can use to store and validate the details of a credit card. Our class will be able to hold the cardholder's name, the card type (mastercard, visa, etc), the card number, and the expiry month and date.

Create a new PHP file called class.creditcard.php. As we walk through the following two pages, copy-paste each piece of code shown to the file and save it.

We start of by defining several card type constants. These values will be used to represent the type of card that our class will be validating:

<?php

define("CARD_TYPE_MC", 0);
define("CARD_TYPE_VS", 1);
define("CARD_TYPE_AX", 2);
define("CARD_TYPE_DC", 3);
define("CARD_TYPE_DS", 4);
define("CARD_TYPE_JC", 5);

Next, we have our class declaration. Our class is called CCreditCard. Note that there is an extra 'C' at the front of the class name intentionally: it's a common programming practice to prefix the name of a class with 'C' to in fact indicate that it is a class.

We also define five member variables, which will be used internally to hold the credit card's name, type, number, expiry month and year respectively:

class CCreditCard
else

To make our CCreditCard class flexible, it accepts several different ways to specify the type of card that is being stored. For example, if we want to add the details of a mastercard to a new instance of our CCreditCard class, then we could pass in the following values for the $type variable of the constructor: "mc", "mastercard", "m", or "1".

We make sure that a valid card type has been passed in, and set the value of our classes $__ccType variable to one of the constant card type values that we defined earlier:

// Make sure card type is valid
switch(strtolower($type))

case 'vs':
case 'visa':
case 'v':
case '2':

case 'ax':
case 'american express':
case 'a':
case '3':

case 'dc':
case 'diners club':
case '4':

case 'ds':
case 'discover':
case '5':

case 'jc':
case 'jcb':
case '6':

default: }

If an invalid card type is passed in, then the default branch of our switch statement will be called, resulting in our script terminating with the die() function.

We can take advantage of PHP's built-in support for regular expressions by using the ereg_replace function to strip out all non-numeric characters from the credit card number:

// Don't check the number yet, just kill all non numerics
if(!empty($num))
else
}
else

We finish off our CCreditCard constructor by making sure that both the expiry month and year are valid, numerical values:

if(!is_numeric($expm) || $expm < 0 || $expm > 12)
else

// Get the current year
$currentYear = date('Y'); settype($currentYear, 'integer');

if(!is_numeric($expy) || $expy < $currentYear || $expy > $currentYear + 10)
else
}

Creating the CCreditCard Class (contd.)
In our CCreditCard class, the only way to set the values of the credit cards details is through the constructor. To actually retrieve the values of our class specific variables ($__ccName, $__ccType, etc), we create several functions, like this:

function Name()

function Type()
case CARD_TYPE_VS:
case CARD_TYPE_AX:
case CARD_TYPE_DC:
case CARD_TYPE_DS:
case CARD_TYPE_JC:
default:
}

function Number()

function ExpiryMonth()

function ExpiryYear()

These functions allow us to retrieve the values of the variables contained within our class. For example, If I created an instance of our CCreditCard class called $cc1, then I could retrieve its expiration month using $cc1->ExpiryMonth().

A common function when working with credit cards is displaying the details that you've captured from that user back to them as a confirmation. For example, if the user entered a credit card number of 4111111111111111, then you might want to only show part of the number to them, such as 4111111111111xxxx. Our CCreditCard class contains a function called SafeNumber, which accepts two arguments. The first is the character to mask the digits with, and the second is the number of digits to mask (from the right):

function SafeNumber($char = 'x', $numToHide = 4)

if($numToHide > 10)

$cardNumber = $this->__ccNum;
$cardNumber = substr($cardNumber, 0, strlen($cardNumber) - $numToHide);

for($i = 0; $i < $numToHide; $i++)

return $cardNumber; }

If we had an instance of our CCreditCard class called $cc1 and the credit card number stored in this class was 4242424242424242, then we could mask the last 6 digits like this:
echo $cc1->SafeNumber('x', 6).

The last function contained in our CCreditCard class is called IsValid, and implements the Mod 10 algorithm against the credit card number of our class, returning true/false.

It starts of by setting two variables ($validFormat and $passCheck) to false: