Credit Card Number Validation in PHP

    Here’s a PHP function to validate credit card numbers using the Luhn algorithm (also known as the “modulus 10” or “mod 10” algorithm), which is commonly used to verify credit card numbers:

    <?php
function validateCreditCard($cardNumber) {
    // Remove any non-digit characters (e.g., spaces, dashes)
    $cardNumber = preg_replace('/\D/', '', $cardNumber);

    // Check if the card number length is between 13 and 19 digits
    if (strlen($cardNumber) < 13 || strlen($cardNumber) > 19) {
        return false;
    }

    // Luhn algorithm
    $sum = 0;
    $shouldDouble = false;

    // Process each digit from right to left
    for ($i = strlen($cardNumber) - 1; $i >= 0; $i--) {
        $digit = $cardNumber[$i];

        // Double every second digit
        if ($shouldDouble) {
            $digit *= 2;
            // If doubling the digit results in a number greater than 9, subtract 9
            if ($digit > 9) {
                $digit -= 9;
            }
        }

        // Add the digit to the sum
        $sum += $digit;

        // Toggle the flag
        $shouldDouble = !$shouldDouble;
    }

    // A valid card number will have a total sum that is a multiple of 10
    return ($sum % 10) === 0;
}

// Example usage:
$cardNumber = "4111111111111111";
if (validateCreditCard($cardNumber)) {
    echo "Credit card number is valid.";
} else {
    echo "Credit card number is invalid.";
}
?>

    Explanation:

    1. Input Sanitization: Removes any non-numeric characters from the input.
    2. Luhn Algorithm: Calculates the checksum to validate the credit card number.
    3. Return Boolean: Returns true if the card number is valid and false otherwise.

    This function works for most standard credit cards (Visa, MasterCard, etc.). However, it does not check specific card types or issuers.