Sieve of Eratosthenes

sieve.php

<?php
// https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes
function sieve(int $n): array {
    $a = array_fill(2, $n - 1, true);
    for ($i = 2, $iMax = floor(sqrt($n)); $i <= $iMax; $i++) {
        if ($a[$i] === true) {
            for ($j = $i ** 2; $j <= $n; $j += $i) {
                $a[$j] = false;
            }
        }
    }
    return array_keys(array_filter($a, function ($e) { return $e; }));
}

// 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97
echo implode(', ', sieve(100));