Luhn PHP

Display

79927398710 is not valid.

79927398713 is valid.

luhn.php

<?php

/**
 * Split number into separate digits.
 *
 * @param int|string $value
 *   This value will be broken into separate digits.
 *
 *   Please note that this will be sanitized to the int type. Letters will be zeroed.
 * @return array
 */
function split_digits($value) {
    return array_map('intval', str_split((string) $value));
}

/**
 * Callback for filtering even indexes.
 *
 * Please note that while this could be called, it is not meant to be used directly.
 *
 * @param $mixed $value
 *   This is ignored.
 * @param $mixed $key
 *   This is converted to an integer and checked if it is even.
 * @return bool
 */
function filter_even_indexes($value, $key) {
    // Unfortunately, we need to increase the index by one to modulo correctly.
    // Could use 1 instead of 0, but that would probably be less clear.
    return (($key + 1) % 2) === 0;
}

/**
 * Callback for filtering odd indexes.
 *
 * Please note that while this could be called, it is not meant to be used directly.
 *
 * @see filter_odd_indexes()
 *   Negates return value.
 *
 * @param $mixed $value
 *   This is ignored.
 * @param $mixed $key
 *   This is converted to an integer and checked if it is odd.
 * @return bool
 */
function filter_odd_indexes($value, $key) {
    return !filter_even_indexes($value, $key);
}

/**
 * Split array by indexes.
 *
 * Technically, this allows for filtering the given array on the callback
 * providing both value and key respectively. The scope for this is limited.
 *
 * @param array $numbers
 *   List of numbers to split.
 * @param callable $callback
 *   Callback for filtering numbers.
 * @return array
 */
function split_indexes(array $numbers, callable $callback) {
    return array_filter($numbers, $callback, ARRAY_FILTER_USE_BOTH);
}

/**
 * Split list of numbers by odd indexes.
 *
 * @param array $numbers
 * @return array
 */
function split_odd_indexes(array $numbers) {
    return split_indexes($numbers, 'filter_odd_indexes');
}

/**
 * Split list of numbers by even indexes.
 *
 * @param array $numbers
 * @return array
 */
function split_even_indexes(array $numbers) {
    return split_indexes($numbers, 'filter_even_indexes');
}

/**
 * Check the number based on luhn algorithm.
 *
 * @param int|string $value
 *   Number to check.
 * @return bool
 *   Whether number passes or fails luhn checksum.
 */
function luhn_checksum($value) {
    $numbers = array_reverse(split_digits($value));
    $total = array_reduce(split_even_indexes($numbers), function($total, $value) {
        return $total + array_sum(split_digits($value * 2));
    }, array_sum(split_odd_indexes($numbers)));
    return ($total % 10) === 0;
}

/**
 * Display whether number is valid based on luhn checksum.
 *
 * @param int|string $value
 *   Number to check.
 * @return void
 */
function is_number_valid($value) {
    if (luhn_checksum($value)) {
        echo sprintf('%d is valid.', $value);
    } else {
        echo sprintf('%d is not valid.', $value);
    }
}

is_number_valid(79927398710);
is_number_valid(79927398713);

Result

79927398710 is not valid.79927398713 is valid.