Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Cell/Coordinate.php
public static function stringFromColumnIndex($columnIndex)
    {
        static $indexCache = [];

        if (!isset($indexCache[$columnIndex])) {
            $indexValue = $columnIndex;
            $base26 = null;
            do {
                $characterValue = ($indexValue % 26) ?: 26;
                $indexValue = ($indexValue - $characterValue) / 26;
                $base26 = chr($characterValue + 64) . ($base26 ?: '');
            } while ($indexValue > 0);
            $indexCache[$columnIndex] = $base26;
        }

        return $indexCache[$columnIndex];
    }
public static function stringFromColumnIndex($columnIndex)
    {
        static $indexCache = [];

        if (!isset($indexCache[$columnIndex])) {
            $indexValue = $columnIndex;
            $base26 = null;
            do {
                $characterValue = ($indexValue % 26) ?: 26;
                $indexValue = ($indexValue - $characterValue) / 26;
                $base26 = chr($characterValue + 64) . ($base26 ?: '');
            } while ($indexValue > 0);
            $indexCache[$columnIndex] = $base26;
        }

        return $indexCache[$columnIndex];
    }
String from column index.

@param int $columnIndex Column index (A = 1)

@return string