Prime Numbers

main.php

<?php

/**
 * Usage:
 * Run the command below to see a list of all prime numbers up to 1000.
 * php main.php 1000
 */

include_once("./PrimeNumbers.php");

if (isset($argv[1])) {
  PrimeNumbers::printPrimeNumbers($argv[1]);
}

PrimeNumbers.php

<?php


// This utility class provides a function that calculates all prime numbers up to a given number.
// Disclaimer: Depending on your system, you may not want to find prime numbers greater than 10000 as
// this operation can take several minutes if not hours.

class PrimeNumbers {
  
  public static function getPrimeNumbers($ceiling) {
    
    // Validate input
    if (!is_numeric($ceiling) || $ceiling < 2) {
      return [];
    }
    elseif ($ceiling < 3) {
      return [2];
    }

    $all_integers = self::getIntegers($ceiling);

    //  Closest integer greater than or equal to the square root of $ceiling
    $square_root = ceil(sqrt($ceiling));

    $prime_numbers = self::getPrimeNumbers($square_root);

    $multiples = self::getMultiples($prime_numbers, $ceiling);

    return array_diff($all_integers, $multiples);
    
  }
  
  public static function getIntegers($ceiling) {
    
    // Get every integer from 0 to N.
    $all_integers = [];
    for ($i = 2 ; $i <= $ceiling; $i++) {
      $all_integers[] = $i;
    }

    return $all_integers;
    
  }
  
  public static function getMultiples($prime_numbers, $ceiling) {
  
    // All multiples less than N, of each element in B:
    $multiples = [];
    foreach ($prime_numbers as $number) {
      $j = 2;
      while (($j * $number) <= $ceiling) {
        if (!in_array($j * $number, $multiples)) {
          $multiples[] = $j * $number;
        }
        $j++;
      }
    }
  
    return $multiples; 
    
  }
    
  public static function printPrimeNumbers($ceiling) {
    if ($ceiling > 10000) {
      print "Input ($ceiling) is too large. Try entering a number with 4 or less digits.\n";
      return false;
    }

    $prime_numbers = self::getPrimeNumbers($ceiling);

    print "Prime numbers up to $ceiling:\n";

    if (empty($prime_numbers)) {
      print "None\n";
      return false;
    }

    foreach ($prime_numbers as $number) {
      print "$number\n";
    }

    print "\n";

    return 1;

  }

}

?>