This function finds postion of nth occurence of a letter starting from offset.

<?php
function nth_position($str, $letter, $n, $offset = 0){
    $str_arr = str_split($str);
    $letter_size = array_count_values(str_split(substr($str, $offset)));
    if( !isset($letter_size[$letter])){
        trigger_error('letter "' . $letter . '" does not exist in ' . $str . ' after ' . $offset . '. position', E_USER_WARNING);
        return false;
    } else if($letter_size[$letter] < $n) {
        trigger_error('letter "' . $letter . '" does not exist ' . $n .' times in ' . $str . ' after ' . $offset . '. position', E_USER_WARNING);
        return false;
    }
    for($i = $offset, $x = 0, $count = (count($str_arr) - $offset); $i < $count, $x != $n; $i++){
        if($str_arr[$i] == $letter){
            $x++;
        }
    }
    return $i - 1;
}

echo nth_position('foobarbaz', 'a', 2); //7
echo nth_position('foobarbaz', 'b', 1, 4); //6
?>