Chuyển index sang dạng chữ alphabet. 0 => A; 1 => B;...25 => Z; 26 => AA...

get_alphabet_from_index.php

<?php
// Get alphabet from index number
// 0 => A; 1 => B
// 25 => Z; 26 => AA
function get_alphabet_from_index($num)
{
    $numeric = $num % 26;
    $letter = chr(65 + $numeric);
    $num2 = intval($num / 26);
    if ($num2 > 0) {
        return get_alphabet_from_index($num2 - 1) . $letter;
    } else {
        return $letter;
    }
}