Credit with PHP #CS50X Harvard

credit.php

<?php

    function main() {

        $len = 0;
        $divisor = 10;
        $ccn;
        // Query user for card details
        do
        {
            $ccn = (int)readline("Enter Card Number: ");
        }
        while ($ccn < 0);

        $cardNumber = $ccn;
        // Computing credit card length
        while($cardNumber != 0)
        {
            $cardNumber = intdiv($cardNumber, 10);
            $len++;
        }
    
        $cardNumber = $ccn;

        // luhns Algorithm

        $sum = 0;

        for ($i = 1; $i <= $len; $i++)
        {
            $digit = $cardNumber % 10;
            

            if ($i % 2 == 0)
            {
                if ($digit * 2 > 9)
                {
                    $sum += $digit * 2 - 9;
                }
                else
                {
                    $sum += $digit * 2;
                }
            }
            else
            {
                $sum += $digit;
            }

            $cardNumber = intdiv($cardNumber, 10);
        }

      
        for ($j = 0; $j < $len - 2; $j++)
        {
            $divisor = $divisor * 10;
        }

        $cardNumber = $ccn; // prevents exhausting original card number in loop
        $firstDigit = intdiv($cardNumber, $divisor);

        // Reduces divisor by 10
        $divisor = round($divisor / 10); // Proper division - like C
        $firstTwoDigits = intdiv($cardNumber, $divisor);


        if ($sum % 10 == 0)     
        {

            if ($len == 15 && ($firstTwoDigits == 34 || $firstTwoDigits == 37))
            {
                printf("AMEX\n");
            }
            else if ($len == 16 && ($firstTwoDigits > 50 && $firstTwoDigits < 56)) 
            {
                printf("MASTERCARD\n");
            }
            else if (($len == 19 || $len == 13 || $len == 16) && ($firstDigit == 4)) 
            {
                printf("VISA\n");
            }

            else 
            {
                printf("INVALID\n");
            }

        }
        else 
        {
            printf("INVALID\n");
        }


    }
    
    // Run Main
    main()
?>

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