LockerProblem.php

<?php
/*
 * Locker Problem
 * Author: Karl Monson
 * URL: https://github.com/karlmonson
 * Resource: http://mathforum.org/library/drmath/view/54242.html
 */
$lockers = array();

for($i = 0; $i <= 1000; $i++){
    $lockers[$i] = 0;
}

for($x = 1; $x <= 1000; $x++){
    for($y = $x; $y <= 1000; $y += $x){
      $lockers[$y] = !$lockers[$y];
    }
}

foreach($lockers as $key => $value){
    if($value == 1){
      echo $key . PHP_EOL;
    }
}