This code uses the Luhn formula to validate your credit card number. This acts as error detection, basicly. It could still be a fake number. I wrote this in PHP because the company is doing more PHP and I figure that I could use the practice. :)

TestCreditCard.php

<?
// What credit card number are we validating?
$CreditCardNumber = '5554172297437101';
// Output the subject
echo $CreditCardNumber;
// Save the credit card number for later
$LastDigit = substr($CreditCardNumber, strlen($CreditCardNumber) - 1);
// Drop the last digit
$CreditCardNumber = substr_replace($CreditCardNumber ,"",-1);
// Reverse the string
$CreditCardNumber = strrev($CreditCardNumber);
// Convert the string to an array
$CreditCardNumber = str_split($CreditCardNumber);
// Multiply all the digits in odd positions by 2
foreach ($CreditCardNumber as $index=>$char){
    if($index % 2 == 0){
        if($char * 2 <= 9){
            // If $char * 2 is less than or equal to 9, just add it
            // Sum the numbers
            $intSumOfNums = $intSumOfNums + $char * 2;
        }else{
            // Else, subtract 9 from it
            // Sum the numbers
            $intSumOfNums = $intSumOfNums + ($char * 2) - 9;
        }
    }else{
        // If we are looking at an even digit, add it to our sum of numbers
        $intSumOfNums = $intSumOfNums + $char;
    }          
}

// The check digit is the amount you need to add to that number to make a multiple of 10
if(($intSumOfNums+$LastDigit) % 10 == 0)
    echo ' is a valid credit card number';
else
    echo ' is not a valid credit card number';

?>