protected function numToLetter($num, $uppercase = FALSE)
	{
		$num -= 1;

		$letter = 	chr(($num % 26) + 97);
		$letter .= 	(floor($num/26) > 0) ? str_repeat($letter, floor($num/26)) : '';
		return 		($uppercase ? strtoupper($letter) : $letter);
	}