public static function int_to_letter($number) {
        $alphabet = [
                '1' => 'A',
                '2' => 'B',
                '3' => 'C',
                '4' => 'D',
                '5' => 'E',
                '6' => 'F',
                '7' => 'G',
                '8' => 'H',
                '9' => 'I',
                '10' => 'J',
                '11' => 'K',
                '12' => 'L',
                '13' => 'M',
                '14' => 'N',
                '15' => 'O',
                '16' => 'P',
                '17' => 'Q',
                '18' => 'R',
                '19' => 'S',
                '20' => 'T',
                '21' => 'U',
                '22' => 'V',
                '23' => 'W',
                '24' => 'X',
                '25' => 'Y',
                '26' => 'Z'
        ];
        if (!is_integer($number) || $number < 1 || $number > count($alphabet)) {
            throw new coding_exception('Only integers between 1 and 26 can be converted to letters.', $number);
        }
        return $alphabet[$number];
    }