Using PHP to find prime numbers within a given range

A prime number is a number which cannot be divided by any number, other than itself and one, and return a whole number. In other words, no matter what number you divide a prime number by, you will have a remainder - unless you divide the number by itself or one.

Here are the prime numbers below 1000.

 

 

And here is the extremely elegant code, which I found on the internet, used to generate them.

 

<?php function getPrimes($finish) { $number = 2; $range = range($number,$finish); $primes = array_combine($range,$range); while($number*$number < $finish){ for($i=$number; $i<=$finish; $i+=$number){ if($i==$number){ continue; } unset($primes[$i]); } $number = next($primes); } return $primes; } ?>

 

The function returns the prime numbers in an array, which can then be used as normal.

Example of how to call the function to find all prime numbers up to 1,000:
<?php
$numbers = getPrimes(1000);
?>

On this page, the results were displayed in a table format using this code - which is by no means perfect (the last row does not have its full complement of cells) but it is designed to be easy to read.

<?php $y=0; echo "<table width='700'><tr>"; foreach($numbers as $number){ if(($y > 0)&&($y % 20 == 0)){ echo "</tr><tr>"; } echo "<td>".$number."</td>"; $y++; } echo "</tr></table>"; ?>