PHP: Sieve of Eratosthenes

sieve-of-eratosthenes.php

<?php
function sieve_of_eratosthenes($max) {
    $flags = array_fill(0, $max, true);
    $flags[0] = $flags[1] = false;

    foreach ( $flags as $index => &$is_prime ) {
        if ( $is_prime ) {
            echo $index . "<br>\n";

            for( $i = $index*$index; $i < $max; $i +=$index )
                $flags[$i] = false;
        }
    }
}