exercism-php/luhn/Luhn.php

<?php

/*
 * By adding type hints and enabling strict type checking, code can become
 * easier to read, self-documenting and reduce the number of potential bugs.
 * By default, type declarations are non-strict, which means they will attempt
 * to change the original type to match the type specified by the
 * type-declaration.
 *
 * In other words, if you pass a string to a function requiring a float,
 * it will attempt to convert the string value to a float.
 *
 * To enable strict mode, a single declare directive must be placed at the top
 * of the file.
 * This means that the strictness of typing is configured on a per-file basis.
 * This directive not only affects the type declarations of parameters, but also
 * a function's return type.
 *
 * For more info review the Concept on strict type checking in the PHP track
 * <link>.
 *
 * To disable strict typing, comment out the directive below.
 */

declare(strict_types=1);

function isValid(string $number): bool {
  $number = str_replace(' ', '', $number);
  if (mb_strlen($number) < 2) {
    return FALSE;
  }
  if (preg_match("/\D/", $number)) {
    return FALSE;
  }
  // At this point, we know the string only contains digits, so ASCII.
  $digits = str_split($number);
  $sum = 0;
  $len = count($digits);
  for ($i = $len - 1; $i >= 0; $i--) {
    $digit = $digits[$i];
    if (($len - $i) % 2 == 0) {
      $digit *= 2;
      if ($digit > 9) {
        $digit -= 9;
      }
    }
    $sum += $digit;
    $sum %= 10;
  }
  return ($sum % 10) === 0;
}