An interesting way to implement the Sieve of Eratosthenes in PHP

primes.php

<?php
    class PrimeIterator implements Iterator {
        private $i;
        private $primes;

        private function generateNextPrime() {
            $candidate = $this->primes[ count( $this->primes ) - 1 ];
            do {
                $candidate += 2;
                $isPrime = true;
                foreach ( $this->primes as $prime ) {
                    if ( $candidate % $prime == 0 ) {
                        $isPrime = false;
                        break;
                    }
                    if ( $prime * $prime > $candidate ) {
                        break;
                    }
                }
            } while ( !$isPrime );
            $this->primes[] = $candidate;
        }
        public function __construct() {
            $this->primes = array( 2, 3 );
            $this->rewind();
        }
        public function rewind() {
            $this->i = 0;
        }
        public function current() {
            if ( !isset( $this->primes[ $this->i ] ) ) {
                $this->generateNextPrime();
            }
            return $this->primes[ $this->i ];
        }
        public function key() {
            return $this->i;
        }
        public function next() {
            ++$this->i;
        }
        public function valid() {
            return true;
        }
    }

    $primes = new PrimeIterator();
    $i = 0;
    foreach ( $primes as $prime ) {
        echo $prime . "\n";
        if ( $i > 10 ) {
            break;
        }
        ++$i;
    }
?>