static function GetColumnLetter($columnNumber)
	{
		// work with 1-based number
		$colNum = $columnNumber + 1;
		$code = "";
		
		if ($colNum > 26)
		{
			// greater than 26 means the column will be AA, AB, AC, etc.
			$left = floor($columnNumber / 26);
			$right = 1 + ($columnNumber % 26);

			if ($left > 26) throw new Exception("Columns exceed supported amount");
			
			$code = chr($left + 64) . chr($right + 64);
		}
		else
		{
			$code = chr($colNum + 64);
		}
		
		return $code;
		
		
	}