exercism-php/sieve/Sieve.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);

enum State {

  case Maybe;
  case Prime;
  case NotPrime;

}

function sieve(int $limit): array {
  $all = [];
  $all[0] = State::NotPrime;
  $all[1] = State::NotPrime;
  for ($i = 2; $i <= $limit; $i++) {
    $all[$i] = State::Maybe;
  }

  for ($i = 2; $i <= $limit; $i++) {
    if ($all[$i] == State::NotPrime) {
      continue;
    }
    $all[$i] = State::Prime;
    for ($j = 2; $j <= $limit / $i; $j++) {
      $all[$i * $j] = State::NotPrime;
    }
  }
  $res = [];
  foreach ($all as $i => $v) {
    if ($v === State::Prime) {
      $res[] = $i;
    }
  }
  return $res;
}