List of all the circular primes under a million

circular_primes.php

<?php
ini_set('memory_limit', '256M');
$maximum = 1000000; // a million if you were wondering..
global $values;

function check_if_circular($prime) {
  global $values;
  $back2back = $prime . $prime; // 7373
  $count = strlen($prime);
  for($i = 0; $i <= $count - 1; ++$i) {
    $number = intval(substr($back2back, $i, $count));
    if ($values[$number] > 0) {
      return FALSE;
    }
  }
  return TRUE;
}

function microtime_float() {
  list($usec, $sec) = explode(" ", microtime());
  return ((float)$usec + (float)$sec);
}

function sieve($uptil) {
  global $values;
  $values = array_fill(0, $uptil+1, -1);
  $values[0] = 1; // 0 is neither prime nor composite
  $values[1] = 1; // 1 is not prime
  $threshold = intval(sqrt($uptil));
  $i = 2;
  while($i <= $threshold) {
    if ($values[$i] > 0) {
      ++$i;
      continue;
    }
    $k = 2;
    $j = $i*$k;
    while($j <= $uptil) {
      $values[$j] = 1;
      $j = $i*(++$k);
    }
    ++$i;
  } 
  $list = array_filter($values, create_function('$elem', 'return ($elem < 0);'));
  $list = array_filter(array_keys($list), "check_if_circular");
  // print_r($list);
  return count($list);
}
$start = microtime_float();
print sieve($maximum) . " in " . (microtime_float() - $start) . " seconds\n";