Algorithm to get the excel-like column name of a number

Here's a nice simple recursive function (Based on zero indexed numbers, meaning 0 == A, 1 == B, etc)...

function getNameFromNumber($num) {
    $numeric = $num % 26;
    $letter = chr(65 + $numeric);
    $num2 = intval($num / 26);
    if ($num2 > 0) {
        return getNameFromNumber($num2 - 1) . $letter;
    } else {
        return $letter;
    }
}

And if you want it one indexed (1 == A, etc):

function getNameFromNumber($num) {
    $numeric = ($num - 1) % 26;
    $letter = chr(65 + $numeric);
    $num2 = intval(($num - 1) / 26);
    if ($num2 > 0) {
        return getNameFromNumber($num2) . $letter;
    } else {
        return $letter;
    }
}

Tested with numbers from 0 to 10000...

Using PhpSpreadsheet (PHPExcel is deprecated)

// result = 'A'
\PhpOffice\PhpSpreadsheet\Cell\Coordinate::stringFromColumnIndex(1);

Note index 0 results in 'Z'

https://phpspreadsheet.readthedocs.io/en/develop/

The correct answer (if you use PHPExcel Library) is:

// result = 'A'
$columnLetter = PHPExcel_Cell::stringFromColumnIndex(0); // ZERO-based! 

and backwards:

// result = 1
$colIndex = PHPExcel_Cell::columnIndexFromString('A');

Indexed for 1 -> A, 2 -> B, etc

function numToExcelAlpha($n) {
    $r = 'A';
    while ($n-- > 1) {
        $r++;
    }
    return $r;
}

Indexed for 0 -> A, 1 -> B, etc

function numToExcelAlpha($n) {
    $r = 'A';
    while ($n-- >= 1) {
        $r++;
    }
    return $r;
}

Takes advantage of the fact that PHP follows Perl's convention when dealing with arithmetic operations on character variables and not C's. Note that character variables can be incremented but not decremented.

This will do to conversion (assuming integer arithmetic), but I agree with the other posters; just use base_convert

function numberToColumnName($number)
{
    $abc = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    $len = strlen($abc);

    $result = "";
    while ($number > 0) {
       $index  = $number % $len;
       $result = $abc[$index] . $result;
       $number = floor($number / $len);
    }

    return $result;
}

Late answer, but here's what I did (for 1==A indexed):

function num_to_letters($num, $uppercase = true) {
    $letters = '';
    while ($num > 0) {
        $code = ($num % 26 == 0) ? 26 : $num % 26;
        $letters .= chr($code + 64);
        $num = ($num - $code) / 26;
    }
    return ($uppercase) ? strtoupper(strrev($letters)) : strrev($letters);
}

Then if you want to convert the other way:

function letters_to_num($letters) {
    $num = 0;
    $arr = array_reverse(str_split($letters));

    for ($i = 0; $i < count($arr); $i++) {
        $num += (ord(strtolower($arr[$i])) - 96) * (pow(26,$i));
    }
    return $num;
}