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File: IsPrime.php

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  Classes of Ahmad Retha  >  Is Prime  >  IsPrime.php  >  Download  
File: IsPrime.php
Role: Class source
Content type: text/plain
Description: IsPrime PHP Class
Class: Is Prime
Check whether a number is prime
Author: By
Last change: False positive bug fix
Date: 2 years ago
Size: 2,258 bytes
 

Contents

Class file image Download
<?php

/**
 * A really fast class to check if a number is a prime number. Basically, it
 * looks at the number and checks its potential for being a prime number step
 * by step. If it passes the fast initial sieve steps then it does the
 * processor-intensive analysis to see if it is a real prime.
 *
 * Usage:
  <code>
  set_time_limit(0);
  include 'IsPrime.php';
  $number=164729;
  echo (IsPrime::test($number)) ? "$number is prime" : "$number is not prime";
  </code>
 *
 * @author Ahmad Retha 31/12/2011
 * @license New BSD
 * @version 2011123100 First!
 * @version 2012010900 Optimization: Replaced '$i < ($num/2)' with '$i < sqrt($num)'; Thanks Jacq Fabien
 * @version 2013062300 Bug fix; Thanks Omar Ramos
 */

class IsPrime
{
   
/**
     * @param int $num
     * @return bool
     */
   
public static function test($num)
    {
       
//-n, 0, 1 not allowed
       
if ($num < 2)
            return
false;

       
//check for single digit primes
       
if (in_array($num, array(2, 3, 5, 7)))
            return
true;

       
//prime numbers end in 1, 3, 7 or 9 no matter how long they are
       
if (!in_array(substr($num, -1), array(1, 3, 7, 9)))
            return
false;

       
//if the number is divisible by 3 or 7 (very common) then it's not prime
       
if ($num%3 == 0 || $num%7 == 0)
            return
false;

       
/*
         * Now find all the numbers up to the square root of potential prime
         * number and test if they divide into it
         */

        //the primes array holds prime numbers to test if they divide into num
       
$primes = array(2, 3, 5, 7);

        for (
$i = 11, $limit = sqrt($num); $i <= $limit; $i++) {

           
//if the number is divisible by a prime number then it's not prime
           
$isPrime = true;
            foreach (
$primes AS $prime) {
                if (
$i%$prime == 0) {
                   
$isPrime = false;
                    break;
                }
            }

           
//check if the increment goes into our number
           
if ($isPrime) {
                if (
$num%$i == 0)
                    return
false;
               
$primes[] = $i;
            }

        }

       
//$num is prime - divisible by itself and 1 only
       
return true;
    }
}