// Created on savesnippets.com · https://savesnippets.com/uMStvYhWPXBrek
<?php
function isValidLuhn(string $number): bool {
    $digits = preg_replace('/\D+/', '', $number);
    $len    = strlen($digits);
    if ($len < 12 || $len > 19) return false;

    $sum = 0;
    for ($i = 0; $i < $len; $i++) {
        $d = (int)$digits[$len - 1 - $i];
        if ($i % 2 === 1) {
            $d *= 2;
            if ($d > 9) $d -= 9;
        }
        $sum += $d;
    }
    return $sum % 10 === 0;
}

var_dump(isValidLuhn('4242 4242 4242 4242'));    // true (Stripe test Visa)
var_dump(isValidLuhn('4242-4242-4242-4243'));    // false (bad checksum)