How can the statement "The check digit is the difference of the sum to the next multiple of 10" be effectively implemented in the code?

To implement the statement "The check digit is the difference of the sum to the next multiple of 10" in code, you need to calculate the sum of the digits in the input number, find the next multiple of 10 greater than this sum, and then subtract the sum from this next multiple of 10 to get the check digit. This can be achieved by converting the input number to an array of digits, summing them up, finding the next multiple of 10, and subtracting the sum from it.

function calculateCheckDigit($inputNumber) {
    $digits = str_split($inputNumber);
    $sum = array_sum($digits);
    $nextMultipleOfTen = ceil($sum / 10) * 10;
    $checkDigit = $nextMultipleOfTen - $sum;
    
    return $checkDigit;
}

$inputNumber = "123456";
$checkDigit = calculateCheckDigit($inputNumber);
echo "Check digit for $inputNumber is: $checkDigit";