luhn_maybe.php

<?php

function luhn2($number) {
    static $sum_table = [[0,1,2,3,4,5,6,7,8,9],[0,2,4,6,8,1,3,5,7,9]];

    $sum = 0;
    $number = (string)$number;
    $i = strlen($number);
    while (--$i) {
        $sum += $sum_table[($i+1) % 2][$number[$i]];
    }   

    return $number[strlen($number)-1] == substr($sum*9,-1,1);
}


function luhn($number) {
    static $sum_table = [[0,1,2,3,4,5,6,7,8,9],[0,2,4,6,8,1,3,5,7,9]];

    $sum = 0;
    $number = (string)$number;
    $i = strlen($number);
    $flip = 0;
    while (--$i)
        $sum += $sum_table[++$flip % 2][$number[$i]];

    return $number[strlen($number)-1] == substr($sum*9,-1,1);
}


var_export(luhn("79927398713"));
var_export(luhn2("79927398713"));