<?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
?>