primepi.php

<?php
/*
  Generates Pi dots at increasing radius of i.
  Ex. gnuplot to render:
  set terminal jpeg size 1000,1000
  set output 'primepi.jpeg'
  unset xtics
  unset ytics
  unset ztics
  unset key
  unset tics
  unset colorbox
  set xrange[-201:201]
  set yrange[-201:201]
  plot 'primepi.dat' using 1:2 pt 20 ps 0.46 lc -1
*/

$primes = [];
$radius = 200;
$fout = fopen( "primepi.dat", "w" );
get_prime( $primes, $radius + 1 );

for( $i = 0; $i <= $radius; $i++ ) {
  $prime = $primes[$i];

  for( $j = 1; $j <= $prime; $j++ ) {
    $y = $i * cos( 2 * M_PI * ( $j/$prime ) );
    $x = $i * sin( 2 * M_PI * ( $j/$prime ) );
    fprintf( $fout, "%5f\t%5f\n", $x, $y );
  }
}

function get_prime( Array &$primes, $n )
{
  ( $n >= 0 ) or die( "Out of range" );

  // Pushes a prime.
  if( ! isset($primes[$n]) ) {
    $count_primes = count($primes);

    while( $count_primes <= $n ) {
      if( $count_primes > 2 ) {
        $c = $primes[$count_primes-1];
        $found_prime = false;
        while( ! $found_prime ) {
          $c += 2;
          $j;

          for( $j = 1; $j < $count_primes; $j++ ) {
            if( $c % $primes[$j] == 0 ) {
              break;
            }
          }
          if( $j == $count_primes ) {
            $found_prime = true;
          }
        }
        $primes[] = $c;
      }
      else {
        $primes = [1,2,3];
      }
      $count_primes = count($primes);
    }
  }
  return $primes[$n];
}