public static function getExcelColumnNumber($letters) {
		$num = 0;
		$arr = array_reverse(str_split($letters));
		$arrSize = count($arr);

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