find-nth.php

<?php
/**
 * Find the index position of the nth occurrence of a string
 *
 * @param string $haystack Frag we looking for
 * @param string $needle Frag we looking for
 * @param int $nth 
 * @return false/int false on not found, else int of strpos
 **/
function find_nth($haystack, $needle, $nth ) {

    preg_match_all("/{$needle}/",$haystack,$capture,PREG_OFFSET_CAPTURE);
    if(isset($capture[0]) && isset($capture[0][ $nth-1 ])) {
        return $capture[0][ $nth-1 ][1];
    }
    return false;
}

$result = find_nth('this is a test string is a', 'is', 4);

if($result !== false) {
    echo "Postion: ".$result;
} else {
    echo "No luck. There aren't that many instances in the haystack.";
}