Determine if secret() is additive or not.

secret.php

<?php
/**
 *
 * Author: Josh Hathcock
 *
 * You are given a function 'secret()' that accepts a single integer parameter and returns an integer. In your
 * favorite programming language, write a command-line program that takes one command-line argument (a number)
 * and determines if the secret() function is additive [secret(x+y) = secret(x) + secret(y)], for all
 * combinations x and y, where x and y are all prime numbers less than the number passed via the command-line
 * argument.  Describe how to run your examples. Please generate the list of primes without using built-in
 * functionality.
 *
 * To execute this script run:
 * $ php secret.php #
 *
 * where "#" is an integer parameter
 *
 */

if (!isset($argv[1]) || !is_numeric($argv[1])) {
    cli("Please enter an integer as a parameter.");
    exit;
}

$limit = intval($argv[1]);

// Secret function
function secret($num) {
  return $num;
}

// Get primes using Sieve of Eratosthenes
function sieve($limit){
  if (!is_int($limit)) {
    return false;
  }

  if ($limit < 1) {
    return false;
  }

  $primes = array(1);

  if ($limit == 1) {
    return $primes;
  }

  $primes = array_fill(0, $limit, true);
  for ($n = 2; $n < $limit; $n++) {
    if ($primes[$n]) {
      for ($i = $n*$n; $i < $limit; $i += $n) {
        $primes[$i] = false;
      }
    }
  }
  return $primes;
}

// Function to test is the 2 numbers are additive
function testSecret($primes) {
  $used = array();

  foreach ($primes as $x) {
    foreach ($primes as $y) {
      if (!in_array($y, $used)) {
        $test1 = secret($x + $y);
        $test2 = secret($x) + secret($y);
        if ($test1 != $test2) {
          return false;
        }
      }
    }
    $used[] = $x;
  }
  return true;
}

// Get the primes of our number
$primes = sieve($limit);

// Now test
if (testSecret($primes)) {
	echo "secret() is additive!\n";
}
else {
	echo "secret() is not additive\n";
}

function cli($message) {
    print $message . PHP_EOL;
}