get the correct excel style column name

exce_column.php

// $c=1 -> A
// $c=27 -> AA
// combine with the row index to get the correct cell identifier,
// e.g.
// $column = _excel_column(26);
// $cell = $columns . '3'; // = Z3

public function _excel_column($c)
{

    $c = intval($c);
    if ($c <= 0) return '';

    $letter = '';

    while($c != 0){
       $p = ($c - 1) % 26;
       $c = intval(($c - $p) / 26);
       $letter = chr(65 + $p) . $letter;
    }

    return $letter;
}