<?php


    function get_character_number($string) {
        $number = 0;
        $string = strtoupper($string);
        $dictionary = array_combine(range('A', 'Z'), range(1, 26));
        for ($pos = 0; isset($string[$pos]); $pos++) {
            $number += $dictionary[$string[$pos]] + $pos * 26 - $pos;
        }
        return $number;
    }
    
    echo get_character_number(''), PHP_EOL; // 0
    echo get_character_number('Z'), PHP_EOL; // 27
    echo get_character_number('AA'), PHP_EOL; // 28

0
26
27