Excel letter columns to number

excel.php

<?php

function number($str) {
    $arr = str_split($str);
    $sum = 0;
    for ($i = count($arr) - 1; $i >= 0; $i--) {
        $idx = ord(strtoupper($arr[$i])) - 64;
        $sum += $idx * (26 ** (count($arr) - $i - 1));
    }

    echo strtoupper($str) . " : " . $sum . "\n";
}