Project Euler Problem 7

Project Euler Problem 7: Find the 10001st prime.

<?php

function is_prime($n)
{
  if ($n < 5) return ($n == 2 || $n == 3);
  for ($p = 2; $p <= 3; $p++)
    if (!($n%$p)) return (($n /= $p) == 1);
  for ($p = 5, $r = floor(sqrt($n)); $p <= $r; $p += 6)
  {
    if (!($n%$p)) return false;
    if (!($n%($p+2))) return false;
  }
  return true;
}

function nth_prime($n)
{
  if ($n < 1) return false;
  if ($n < 3) return $n+1;
  for ($n -= 2, $p = 5 ; ; $p += 6)
  {
    if (is_prime($p) && !--$n) break;
    if (is_prime($p+2) && !--$n)
    {
      $p += 2;
      break;
    }
  }
  return $p;
}

echo nth_prime(10001);

?>