spreadsheetLetters.php

/**
 * Converts a number to A,B,C,...,X,Y,Z,AA,AB,AC,...,ZZ,AAA,AAB, etc.
 *
 * @param int $int must be at least 1
 * @return string
 */
function spreadsheetLetters(int $int): string {
    if ($int < 0) throw new TypeError('$int must be at least 0');

    $numeric = $int % 26;
    $letter = chr(65 + $numeric);
    $int2 = intval($int / 26);
    if ($int2 > 0) {
        return (__FUNCTION__)($int2 - 1) . $letter;
    } else {
        return $letter;
    }
}