PHP Luhn Validator (from Symfony)

LuhnValidator.php

<?php

/**
 * Validates a value using the LUHN Algorithm.
 * @param string $value Value to validate. EG: Credit card number
 * @return bool
 */
function isLuhn($value)
{
    if (!is_string($value) || !ctype_digit($value)) {
        return false;
    }

    $checkSum = 0;
    $length = strlen($value);

    for ($i = $length - 1; $i >= 0; $i -= 2) {
        $checkSum += $value{$i};
    }

    for ($i = $length - 2; $i >= 0; $i -= 2) {
        $checkSum += array_sum(str_split($value{$i} * 2));
    }

    if (0 === $checkSum || 0 !== $checkSum % 10) {
        return false;
    }

    return true;
}