Get A Number In Prime Series By Index; The nTH Prime.

nth_prime.php

<?php
function getPrimingSeriesIndex($index)
{
    if ( $index <= 0 )
        return false;


    if ($index == 1)
        return 2;


    $offset = 1;

    do {
        (! isset($primeNum)) ? $primeNum = 3 : $primeNum++;

        $flagNumDivided = 0;
        for ( $i = 1; $i <= $primeNum; $i++) {
            if ( ($primeNum % $i) == 0 ) {
                if (++$flagNumDivided > 2)
                    // sure; its not prime
                    break;
            }
        }

        if ($flagNumDivided <= 2)
            $offset++;

        if ($offset == $index)
            return $primeNum;

    } while(1);
}