Finding primes in PHP with a lazy collection library.

primes.php

<?php

/**
 * @file
 * Example/demo file.
 *
 * @see https://github.com/loophp/collection
 */

declare(strict_types=1);

include 'vendor/autoload.php';

use loophp\collection\Collection;
use function in_array;
use const INF;

/**
 * Get the divisor of a given number.
 *
 * @param float $num
 *   The number.
 * @param int $start
 *   The start.
 *
 * @return \Traversable
 *   The divisors of the number.
 */
function factors(float $num, int $start = 1): Traversable
{
    if (0 === $num % $start) {
        yield $start => $start;

        yield $num / $start => $num / $start;
    }

    if (ceil(sqrt($num)) >= $start) {
        yield from factors($num, $start + 1);
    }
}

/**
 * Check if a number is a multiple of 2.
 *
 * @param $value
 *   The number.
 *
 * @return bool
 *   Whether or not the number is a multiple of 2.
 */
$notMultipleOf2 = static function ($value): bool {
    return 0 !== $value % 2;
};

/**
 * Check if a number is a multiple of 3.
 *
 * @param $value
 *   The number.
 *
 * @return bool
 *   Whether or not the number is a multiple of 3.
 */
$notMultipleOf3 = static function ($value): bool {
    $sumIntegers = static function ($value): float {
        return array_reduce(
            mb_str_split((string) $value),
            static function ($carry, $value) {
                return $value + $carry;
            },
            0
        );
    };

    $sum = $sumIntegers($value);

    while (10 < $sum) {
        $sum = $sumIntegers($sum);
    }

    return 0 !== $sum % 3;
};

/**
 * Check if a number is a multiple of 5.
 *
 * @param $value
 *   The number.
 *
 * @return bool
 *   Whether or not the number is a multiple of 5.
 */
$notMultipleOf5 = static function ($value): bool {
    return !in_array(mb_substr((string) $value, -1), ['0', '5'], true);
};

/**
 * Check if a number is a multiple of 7.
 *
 * @param $value
 *   The number.
 *
 * @return bool
 *   Whether or not the number is a multiple of 7.
 */
$notMultipleOf7 = static function ($value): bool {
    $number = $value;

    while (14 <= $number) {
        $lastDigit = mb_substr((string) $number, -1);

        if ('0' === $lastDigit) {
            return true;
        }

        $number = (int) abs((int) mb_substr((string) $number, 0, -1) - 2 * (int) $lastDigit);
    }

    return !(0 === $number || 7 === $number);
};

/**
 * Check if a number is a multiple of 11.
 *
 * @param $value
 *   The number.
 *
 * @return bool
 *   Whether or not the number is a multiple of 11.
 */
$notMultipleOf11 = static function ($value): bool {
    $number = $value;

    while (11 < $number) {
        $lastDigit = mb_substr((string) $number, -1);

        if ('0' === $lastDigit) {
            return true;
        }

        $number = (int) abs((int) mb_substr((string) $number, 0, -1) - (int) $lastDigit);
    }

    return !(0 === $number || 11 === $number);
};

/**
 * Check if a number have more than 2 divisors.
 *
 * @param $value
 *   The number.
 *
 * @return bool
 *   Whether or not the number has more than 2 divisors.
 */
$valueHavingMoreThan2Divisors = static function ($value): bool {
    $i = 0;

    foreach (factors($value) as $factor) {
        if (2 < $i++) {
            return false;
        }
    }

    return true;
};

$start = microtime(true);
$primes = Collection::range(9, INF, 2) // Count from 10 to infinity
    ->filter($notMultipleOf2) // Filter out multiples of 2
    ->filter($notMultipleOf3) // Filter out multiples of 3
    ->filter($notMultipleOf5) // Filter out multiples of 5
    ->filter($notMultipleOf7) // Filter out multiples of 7
    ->filter($notMultipleOf11) // Filter out multiples of 11
    ->filter($valueHavingMoreThan2Divisors) // Filter out values having more than 2 divisors.
    ->prepend(2, 3, 5, 7) // Add back digits that were removed
    ->normalize() // Re-index the keys
    ->limit(100); // Take the 100 first prime numbers.

print_r($primes->all());
$time = microtime(true) - $start;
var_dump($time);