$prime = array();

$max = 200000;

 

function prime($input) {

global $prime;

return $prime[$input];

}

 

function getPrimeByOrder($order) {

global $prime;

for($a = 0, $walk = 1; $a < $order ; $a += $prime[$walk], $walk++);

return $walk - 1;

}

 

for ($a = 2 ; $a <= $max ; $prime[$a] = 1, $a++);

 

for ($a = 2 ; $a <= sqrt($max); $a++) {

if ($prime[$a] == 0) continue;

for ($b = ($a+$a) ; $b <= $max ; $prime[$b] = 0, $b+=$a);

}

 

echo getPrimeByOrder(10001);