<?php

function find_nth_prime($x = 6) {

          // Find the $xth prime.

          $i = 3;

          $a = $b = 1;

          while ($i <= $x) {

                    if ($b == 1) {

                              $r = 6 * $a - 1;

                              $b = 0;

                    } else {

                              $r = 6 * $a + 1;

                              $b = 1;

                              $a = $a + 1;

                    }

                    $c = pow($r, .5);

                    $d = 1;

                    $e = 3;

                    while ($e <= $c) {

                              if ($r % $e == 0) {

                                        $d = 0;

                              }

                              $e = $e + 2;

                    }

                    if ($d == 1) {

                              $i = $i + 1;

                    }

          }

          return $r;

}

echo find_nth_prime(10001); // 6.683 seconds

?>