The below code calculates pi to 13 decimals using PHP and the Nilakantha Series. We loop through the series 15468 times to get an accurate result.

<?php
$pi = 3;
$counter = 2;
$direction = 1;
for ($x = 0; $x <= 15468; $x++) {
    if ($direction == 1){
        $pi = $pi + (4 / ($counter * ($counter + 1) * ($counter + 2)));
        $counter = $counter + 2;
        $direction--;
    } else {
        $pi = $pi - (4 / ($counter * ($counter + 1) * ($counter + 2)));
        $counter = $counter + 2;
        $direction++;
    }
}
echo $pi;
?>